High-speed energy-efficient memristor confined in sub-5 nm space with elemental oxygen reservoir layer

Domain: chemistry
Published: 2026-03-18
Editorial outcome: major_revision

Abstract
Random migration of oxygen vacancies (V ) leads to unpredictable formation and rupture of
conductive filaments (CFRs) in oxide-based memristors. In this work, an atomically flat 4.5 nm
hafnium oxide (HfO ) Aswitching layer and a 3.5 nm elemental oxygen reservoir (EOR) layer are
x
confined between two-dimensional HfS and MoS layers, ensuring a homogeneous electric
2 2
field distribution. The migration and redistribution of V within the ultrathin HfO switching
O x
layer enable the memristive behavior of the device. The EOR-based memristors achieve high
set/reset transition speeds of 8 ns and 15 ns, respectively. The electroneutral EOR layer
interacts with V in the HfO switching layer and, together with the HfO tunnel layer above the
O x x
HfS , forms a barrier to suppress the high-resistance state current. Reliable endurance up to 105
2
cycles, and long retention up to 105 s are simultaneously obtained. Finally, a high recognition
accuracy of 97.0% is achieved, demonstrating potential for low-power neuromorphic
computing applications.

Introduction
Memristors are two-terminal devices featuring resistance realized through mobile species,
typically oxygen vacancies (V ), in an oxide-based switching layer. These species can be driven
O
back and forth to create conductive filaments (CFs)1-5. Under an electric field, the drift of mobile
V causes the memristor to switch from a high-resistance state (HRS) to a low-resistance state
O
(LRS). Thus, the resistive switching behavior of oxide-based memristors is governed by the growth
and rupture of nanoscale CFs6-9. The LRS in such devices is determined by the formation of a
percolation path between the electrodes, while data retention refers to the time required for these
CFs to dissipate under ambient conditions10-12. However, irregular migration paths and the
continuous accumulation of V impact the uniformity of the coSnductive path during the set process,
O
E
and increase variation in the rupture process during the reset process13. The expanded space for V
O
R
migration and redistribution during cycling gives rise to uncertainties and complications, resulting
P
in eventual failure or irreversibly degraded memri stive switching14. Meanwhile, as the set/reset
N
speed is limited by the electrical charging time and the random migration of ions, it is challenging
I
to control the diffusion coefficients or path s of ions within the switching oxide thin films8,15,16.
E
Although ultrathin switching layers ofLfer an alternative route to minimize power consumption and
C
increase operation speed, the morphology and scale of the CFs are significantly affected when the
I
thickness is scaled down, makTing the control of CF growth/rupture more difficult. The presence of
R
residual CFs generates a large HRS current, leading to additional static power consumption.
A
Especially when the switching layer thickness is below 10 nm, surface roughness fluctuations
impact the growth/rupture of CFs and increase the uncertainty of the switching process. Therefore,
it is urgent to design ultrathin material stacks with a uniform structure and desirable interfaces to
ensure a homogeneous electric field distribution across the switching layer, which can provide
predictable migration paths2,8,16. Additionally, in forming-free oxide-based memristors, previous
works have primarily focused on modulating oxygen content through bilayer designs11,17. However,
the uncontrolled presence of other unwanted defects leads to charge trap formation, which in turn
results in high leakage currents and poor cycling stability14,15,18,19. Furthermore, a defective metal-
insulator interfacial layer may induce defects to extend into the switching layer, creating parasitic
series connections17. Consequently, it is difficult to maintain a high on-off ratio and low leakage
current while reducing the thickness of the switching layer20-23. Notably, by confining the
stochastic switching layer between two atomically flat interfaces, stochastic diffusion at the
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interfaces can be suppressed during operation under electrical fields8,15. Here, we demonstrate that
an electroneutral elemental oxygen reservoir (EOR) layer can form an atomically flat interface
with an ultrathin oxide layer. It acts as a tunneling barrier to block the stochastic migration of V
O
and suppress leakage current15. As a result, the integration of an EOR layer with the switching
oxide layer enables highly stable memristor operations with ultralow power consumption by
suppressing leakage current.

In this article, a 4.5 nm hafnium oxide (HfO ) switching layer and a 3.5 nm electroneutral
x
EOR layer are sandwiched between HfS and MoS flakes to form an ultrathin memristor structure.
2 2
The ultrathin HfO switching layer features a uniform structure and is derived from the HfS₂ flake
x S
through an ozone treatment, utilizing the atomically flat surfacSe of the MoS
2
flake as a substrate.
This configuration enables ultrafast and low-power memristEive operations. The memristor exhibits
R
a high on-off ratio of 1.47×109 and an ultralow average HRS current of 1.52 fAμm-2 along with
P
ultra-short set/reset transition times (8 ns/15 ns) . Furthermore, we demonstrated handwriting
N
recognition using a memristor-based neural network model on the Modified National Institute of
I
Standards and Technology (MNIST) dataset . By adjusting the pulse amplitude to update the weight
E
parameters, we achieved a recognitiLon accuracy of 97.0%. This work paves the way for the
C
development of high-performance, energy-efficient computing and intelligent systems.

Results
Fabrication of elemental oxygen reservoir-based transistor
The fabrication processes are depicted in Fig. 1a-c. In brief, a few-layered MoS flake is
2
mechanically exfoliated using Scotch tape and transferred onto an Cr/Au electrode on a SiO / p+-
2
Si substrate. Subsequently, a 2D HfS flake with a thickness of ~12 nm is stacked onto the MoS
2 2
flake to assemble an HfS /MoS van der Waals junction. Then, an ozone treatment is introduced
2 2
to convert the HfS flake into an HfO /HfS /HfO functional stack. The oxidation process of HfO
2 x 2 x x
can be precisely controlled by adjusting the oxidation time, and the evolution of the HfS flake
2
under ozone treatment is illustrated in Supplementary Fig. 1. An atomic force microscopy (AFM)
image indicates a smooth surface, and the thickness variation of the stack across different ozone
treatment durations is presented in Supplementary Fig. 2. The increased height of the
HfO /HfS /HfO layer stems from the transition of the layered crystalline HfS flake into an
x 2 x 2
amorphous oxide layer. Finally, photolithography, thermal evaporation, and lift-off processes are
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carried out in sequence to form the top contact leads, as shown in Fig. 1c. An EOR layer is formed
between the HfO switching layer and the bottom MoS flake, which originates from the van der
x 2
Waals contact of the MoS /HfS stack. Fig. 1d presents the SEM image of the as-fabricated vertical
2 2
Au/MoS /HfO /HfS /HfO /Au EOR-based memristor, in which the active area of each memristor
2 x 2 x
is 4.2 µm × 4.2 µm, defined by the overlap region between the top and bottom electrodes. Since a
mild ozone treatment is employed as a substitute for a relatively high-energy oxidation process, a
mean roughness of 2.61 nm is obtained after oxidation for 1 hour, as shown in Fig.1e. Although
ozone treatment involves reactive species and may alter the MoS surface, HfS flake fully
2 2
overlaps the MoS flake and the contact regions, suppressing unintentionally introduced defects
2 S
and the influence of ultraviolet light illumination on MoS
2
. ConSsequently, the obtained HfO
x
/MoS
2
interface is atomically steep. The Raman spectra of differeEnt regions of the fabricated memristor
R
are shown in Fig. 1f. The degraded E /A phonon modes confirm the transition to amorphous
g 1g
P
HfO
x
, while the slightly shifted E1
2 g
/A
1g
peaks of MoS
2
can be attributed to the doping effect
N
induced by the EOR layer. We conducted a detailed depth-resolved X-ray photoelectron
I
spectroscopy (XPS) characterization of the E OR-based memristors and analyzed the bonding state
E
of oxygen in different layers, as showLn in Fig. 1g. The content of high binding energy oxygen,
C
corresponding to O-O bonds in the EOR-based memristors, is relatively high on the HfO surface,
x
I
which may be caused by surfTace adsorbed oxygen, as shown in Supplementary Fig. 3a. Since the
R
XPS spectral signal is collected within a depth of ~5 nm, the Mo and S contents simultaneously
A
increase after 5 minutes of etching, as shown in Fig. 1g, suggests that the XPS spectra obtained
when the etching time reaches 6 minutes mainly originate from the EOR layer. The S concentration
peaks at an etching time of 2 minutes, indicating that the spectrum obtained after 3 minutes of
etching represents the HfO switching layer. The intensity of V rises in the HfO switching layer,
x O x
as shown in Fig. 1h. Notably, the O-O signal is dominant in the EOR region, while the V and Hf-
O
O bonds significantly decrease, as shown in Fig. 1i. Therefore, the EOR layer is mostly composed
of O-O bonds. Moreover, O-O related species, such as O 2-, O -, and O , possess low bond orders
2 2 3
and are metastable. Thus, they are highly prone to gaining or losing electrons to achieve a more
stable O electronic configuration. Metallic hafnium is observed in the HfO switching layer, as
2 x
shown in Supplementary Fig. 3b. Typically, metallic hafnium promotes the formation of local V -
O
related CFs, thereby reducing the forming voltage and even enabling forming-free operations.
Furthermore, as shown in Supplementary Fig. 3c, the absence of Mo 3d features indicative of Mo
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oxidation in the XPS spectrum suggests that no Mo-O-related species were formed. Consequently,
the EOR layer is mainly composed of electroneutral O .
2

Electrical performance
To further characterize the switching capability of the vertical EOR-based memristor, electrical
measurements are conducted using quasistatic d.c. sweeping and dynamic electrical pulses.
The
MoS /HfS stack is fabricated in a nitrogen-filled glove box via mechanical exfoliation. Due to the
2 2
low oxygen vacancy concentration in the unoxidized state, negligible memristive behaviors are
observed, as shown in Supplementary Fig. 4. As the thickness of the HfO switching layer is
x
S
determined by the ozone treatment time, Fig. 2a presents the oxidation-time-dependent current-
S
voltage (I-V) curves of the vertical EOR-based memristoErs, which clearly demonstrate stable
R
bipolar resistive switching characteristics. A systematic reduction in the off-state current density
P
is observed with increased oxidation time. Accumulated oxygen tends to adsorb onto the HfO
x
N
surface and can recombine with V in HfO near the EOR layer. Consequently, the effective
O x
I
thickness of the HfO
x
switching layer may b e thinner than the actual physical thickness. Combined
E
with the metallic hafnium content and high V concentration, the ultrathin HfOₓ utilizes the
L O
intrinsic electronegativity gradienCt to drive oxygen redistribution, allowing the memristor to
I
operate in a forming-free mode. The slightly inconsistent set voltages may be attributed to
T
thickness variations in theR HfO
x
flakes during the oxidation process (Fig. 2b). Since the process of
A
oxidizing HfS into HfO is not self-limiting, increasing the oxidation time (ozone treatment time)
2 x
increases the HfO thickness. However, this leads to a continuous increase in the HRS current after
x
40 min, as shown in Fig. 2b. This is mainly because as the thickness increases, the number of
defects also increases, significantly raising the probability of tunneling through the EOR layer.
Moreover, the values of V and V also constantly increase when the ozone treatment
SET RESET
exceeds 40 min, which is mainly because the increased thickness requires a higher operating
voltage. Therefore, reducing the thickness of the switching layer can decrease the power
consumption of the device by lowering the operating voltage. The film thickness reaches
saturation after 80 minutes of ozone treatment. However, prolonged exposure leads to the
degradation of the surface morphology, resulting in a marginal increase in the SET and RESET
operating voltages. The LRS current is primarily determined by the confined CFs formed by V
O
within HfO regions, so the LRS current remains stable. Typical set/reset cycles of 100 vertical
x
EOR-based memristors from four 5 × 5 memristor arrays are presented in Fig. 2c. The
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corresponding I-V curves with absolute current values are provided in Supplementary Fig. 5. Since
the electroneutral EOR layer functions as a barrier for leakage current, the potential defect-assisted
current in the HRS state is suppressed. According to the statistical cumulative probability
distribution of current at HRS and LRS in Fig. 2d, an ultra-low average HRS current density of
1.52 fA µm-2 and a high average on-off ratio of 1.47 × 109 at a read voltage (V ) of 0.1 V are
Read
simultaneously achieved (Supplementary Fig. 6a). Statistical analysis reveals that the set and reset
voltages (V and V ) follow Gaussian distributions centered at 1.2 V and −0.8 V, respectively
SET RESET
(Supplementary Fig. 6b).

The electroneutral EOR layer also contributes to the robust retention performance of the
fabricated memristor, as shown in Fig. 2e. At V
READ
= 0.1S V, stable HRS/LRS currents are
maintained for 105 s at room temperature without degradatioEn. Testing stability at high temperature
R
is necessary to illustrate the performance of the EOR-based memristor. As shown in
P
Supplementary Fig. 7, reliable and consistent switc hing characteristics over 105 s are obtained at
N
85 °C and 125 °C, respectively. Typical endurance performance of the fabricated memristors at
I
room temperature is shown in Fig. 2f. Und er continuous pulses of ±1.5 V/200 μs, the LRS and
E
HRS exhibited remarkably stable swLitching over 105 cycles, with both HRS and LRS currents
C
remaining constant and showing minimal fluctuations. Fig. 2g presents the detailed response under
I
consecutive pulse excitation.T Although a 100 ns pulse is insufficient to drive the LRS current of
R
the EOR memristor to a large saturation value, such as approaching the compliance current, an on-
A
off ratio of ~104 is still obtained. This indicates that such a short pulse is sufficient for the growth
of CFs and that the device can operate normally under short pulse conditions without performance
degradation after 105 cycles (Supplementary Fig. 8). To investigate the kinetics of forming-free
switching, we performed pulse-width-dependent experiments, varying the pulse width from 10 ns
to 100 ns. As shown in Supplementary Fig. 9, the LRS current increases with the increase of pulse
width. The switching time of the device is evaluated by V-t and I-t synchronous curves measured
in pulse mode. Fig. 2h shows the oscilloscope waveforms of the applied voltage pulses (±1.5
V/100 ns) and the measured current during set/reset operations. A constant bias of 0.1 V (read
voltage) is used to measure the current. The induced current takes 8 ns to increase from 10% to
90% of its maximum value, which can be characterized as the set transition time of the EOR-based
memristor. The reset pulse gradually pulls down the current as filament dissolution proceeds, and
a reset transition time of 15 ns is obtained. The performance parameters of the fabricated memristor
demonstrate a high on-off ratio and an ultralow off-current compared with previously reported
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ultrathin memristors, as shown in Fig. 2i. This work presents a feasible method to fabricate
atomically thin an HfO switching layer without invasive sputtering or atomic layer deposition.
x
Moreover, in the fabricated memristor, no forming process is required to initiate resistive switching
and create CFs, making it advantageous in terms of forming-free operation compared to previously
reported ultrathin oxide memristors, as shown in Supplementary Table 110,17,24-33.

Operation mechanism
To investigate the switching mechanism and ultralow HRS current of the fabricated EOR-based
memristor arrays, electron energy loss spectra (EELS) line scans are carried out across the EOR-
S
based memristor, enabling direct visualization of the oxygen distribution in the vertical stack. This
S
visualization elucidates the structural and compositional vaEriations of the device. This distinct O
R
K edge at 532 eV reveals the partial density of states of elemental oxygen, and the shaded region
P
represents the O K partial density of states summed from different layers, indicating the presence
N
of an EOR layer between the MoS and the switching HfO layer, as shown in Fig. 3a. Energy loss
2 x
I
near edge structure (ELNES) spectra are sho wn in Supplementary Fig. 10a. The O K-edge of HfO
x
E
is dominated by two peaks labeled p1 and p2, centered at 534.5 eV and 538.1 eV, respectively.
L
Typically, p1-p2 peak splitting is aCbsent in transition metal oxide films with excess oxygen. The O
I
K-edge of the EOR layer exhibits an obvious single peak at 538.3 eV. Therefore, these results
T
confirm the oxygen-rich nRature of the HfO
x
and EOR layer in the stack34-37. Transmission electron
A
microscopy (TEM) and energy dispersive spectroscopy (EDS) mapping are shown in Fig. 3b, c,
and the thicknesses of the switching HfO layer and the EOR layer are 4.5 nm and 3.5 nm,
x
respectively. The EOR layer originates from the van der Waals contact of the HfS /MoS interface,
2 2
where the oxygen-rich condition leads to oxygen atoms to accumulate at the interface, allowing
the HfS flake to be partly oxidized from both sides. Since the atomically flat surface of MoS
2 2
provides an flat platform for the growth of the HfO switching layer, the vertical architecture
x
exhibits atomic flatness at the interface (Supplementary Fig. 10b). This minimizes reliability issues
caused by thickness fluctuations, especially in the case of sub-10 nm thickness. Moreover, this
interface limits unwanted charge trapping, which helps stabilize the local electric field distribution
and improves the uniformity of the switching process. Therefore, the ultra-flat HfO switching and
x
EOR layers ensure a homogeneous electric field distribution, enabling stable resistive switching
operations.

The bipolar resistive switching characteristics of the EOR-based memristor originate from
the synergistic interplay of V migration within the HfOₓ switching layer located between the HfS
O 2
and EOR layers, as schematically illustrated in Fig. 3d. Briefly, a positive bias (set) creates an
electric field that induces a simultaneous soft breakdown via field-driven V percolation within
O
the vacancy-rich HfO switching layer. As a result, CFs are formed, and the memristor is set to the
x
LRS. The opposite effect occurs when applying a negative voltage (reset). Oxygen ions at the
HfO
x
/EOR interface recombine with V
O
, rupturing the CFs, and the memristor is reset to the HRS
with negligible residual filaments. The HfO tunneling layer above the HfS acts as a series
x 2
tunneling barrier to suppress crosstalk, helping to mitigate excessive current flow. Sneak-path
S
analysis is presented in Supplementary Fig. 11, where theS voltage pulsing scheme involves
applying a designated voltage to the selected cell and smaEller voltages to the half-selected cells
R
due to the standby voltage scheme. The selected cell experiences conductance changes under
P
external pulses, while other regions of the crossba r arrays remain largely unaffected. Moreover,
N
intended digital patterns are successfully stored and read out, as shown in Supplementary Fig. 12,
I
demonstrating that the readout of neighbori ng cells is not disturbed by sneak currents. This result
E
confirms that crosstalk in the proposedL memristor array is effectively suppressed and that each cell
C
can be individually addressed and read without unintended interference from adjacent devices.
I
Since the MoS
2
and HfS
2
layTers present high energy barriers for V
O
migration, and the EOR layer
R
prevents oxygen drift and diffusion, relatively thin CFs are formed in the HfO layer. This helps
x
A
reduce overcurrent stress, facilitating low-current operation. The HfO switching layer and EOR
x
layer are confined within a nanoscale space, and stochastic diffusion at the interface is effectively
suppressed.

The I-V curves in Fig. 3e demonstrate bipolar nonvolatile switching from 77 K to 300 K,
revealing that temperature significantly influences the resistive switching behavior. As shown in
Fig. 3f, the slope of the fitting lines in the low voltage region of the LRS demonstrates linear I ∝
V1 behavior, which is consistent with Ohmic conduction through percolating CFs formed under
electric field-driven V migration. Defect-related traps in the switching oxide layer contribute to
O
a slightly higher slope (greater than 1). The slope of the fitting lines increases to ~2 at higher
voltage, which is correlated with an increase in trap concentration within the band gap. Since
Child’s Law (I ∝ V2) applies to the space charge limited conduction (SCLC) mechanism, SCLC
and Ohmic conduction coexist to govern the HRS conduction mechanism, whereas Ohmic
conduction dominates the LRS conduction mechanism. Specifically, conduction in the HRS
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transitions from Ohmic behavior (I ∝ V) at low biases to SCLC behavior at higher biases,
following Child’s law. The transition to SCLC signifies the dominance of injected carriers over
the intrinsic defect-induced charge density. Fig. 3g plots the set and reset voltages versus
temperature. Decreased migration efficiency hinders the formation and dissolution of CFs at low
temperatures, resulting in higher switching voltages for the memristor. The temperature-dependent
current plots for HRS and LRS at V
Read
= 0.1 V are shown in Fig. 3h. Based on the Arrhenius
plot with linear fitting shown in Supplementary Fig. 13, the barrier height is extracted to be 15.4
meV. Owing to the uniform and ultrathin HfO switching layer confined within a sub-10 nm space,
x
a homogeneous electric field distribution is realized across the device. The EOR layer and HfS
S 2
flake act as diffusion barriers to inhibit the random diffusion Sof V
O
, leading to robust endurance
and retention performance of the EOR-based memristors.

Artificial neural networks
The development of biologically plausible artificial neural networks (ANNs) necessitates the
precise emulation of synaptic plasticity thro ugh memristive architectures, as shown in Fig. 4a. A
memristor crossbar array is usually employed to perform vector-matrix multiplication for pattern
classification. The detailed multilaCyer perceptron (MLP) structure is shown in Fig. 4b, comprising
784 input neurons, 300 hidden neurons, and 10 output neurons, all interconnected by device-
simulated synapses. DigitRal images of different handwritten digits are imported into the first layer
of neurons and are processed by the MLP algorithm. This synaptic analog enables the construction
of a network with backpropagation-tunable weights, as shown in Fig. 4c, which eliminates von
Neumann bottlenecks through analog vector-matrix multiplication within the memristor array.
Synaptic validation commenced with paired-pulse experiments. Fig. 4d, e illustrates the paired-
pulse facilitation (PPF) and paired-pulse depression (PPD) characteristics, respectively. The PPF
ratio decreases with increasing time intervals between pulses, following a double exponential
decay function, while the PPD ratio increases with the time interval, also following a double
exponential function. These bidirectional time-dependent responses mirror biological short-term
plasticity governed by presynaptic calcium dynamics. The memristor exhibits frequency-
dependent current modulation under varying pulse train frequencies (102-104 Hz), confirming its
adaptability to various stimulation patterns (Fig. 4f). This adaptive signal transduction capability
underscores the device's potential for implementation in complex neural network applications.
This memristor exhibits long-term potentiation (LTP) under high-frequency activity and long-term
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depression (LTD) under low-frequency activity, as shown in Fig. 4g, h. The simulation results, as
shown in Fig. 4i, indicate that the trained neural network achieves a recognition accuracy of 97.0%
for the device using non-identical pulses and 91.5% for the device using identical pulses,
approaching the ideal simulation value of 97.4% based on the MNIST dataset. Therefore, the EOR-
based memristor array shows great potential for neural network systems.

By taking advantage of the atomically flat interface of 2D materials, a uniform and ultrathin
HfO switching layer is fabricated between HfS and MoS Consequently, the migration of oxygen
x 2 2.
vacancies is confined within a sub-10 nm space, ensuring a homogeneous electric field distribution.
The EOR layer acts as a diffusion barrier and facilitates the rupture of CFs in the HRS, enabling
S
ultrafast operation and low power consumption. Furthermore,S the EOR-based memristors exhibit
desirable retention and endurance performance. Moreover, fEorming-free operation is achieved due
R
to the ultrathin nature, high V concentration, as well as the presence of metallic hafnium in the
O
P
switching layer. Simulation results demonstrate that the EOR-based memristor array obtains a high
N
recognition accuracy, indicating its potential for low-power memory and neuromorphic computing
I
applications.

Methods
Device Fabrication
The EOR-based memristor arrays were fabricated on a SiO /p⁺-Si substrate. The bottom Au
2
electrodes (50 nm) with a Cr adhesion layer (10 nm) were prefabricated using standard mask
photolithography, thermal evaporation, and lift-off processes. Multilayer MoS flakes were
2
mechanically exfoliated from a bulk crystal (Shanghai Onway Technology Co., Ltd) using Scotch
tape and then transferred onto the prefabricated Au electrodes. Subsequently, HfS₂ flakes with a
thickness of ~12 nm, also mechanically exfoliated, were stacked onto the MoS₂ flakes to form an
HfS₂/MoS₂ van der Waals heterostructure. The functional HfOₓ switching layer and the elemental
oxygen reservoir (EOR) layer were created by subjecting the HfS₂/MoS assembly to a mild ozone
2
treatment. The thickness of the resulting HfOₓ layer was precisely controlled by adjusting the
duration of the ozone exposure. The ozone was produced by ultraviolet light in a reaction chamber
(Model BZS250GF-TS) pre-charged with oxygen gas under ambient pressure conditions. A 300
W ultraviolet lamp was employed as the excitation source, delivering an ultraviolet intensity of 50
mW cm-2. Meanwhile, thermal irradiation regulated the substrate temperature at a mild 310 K.
Methyl methacrylate/poly methyl methacrylate (MMA/PMMA) bilayer was employed as the
photoresist material, and EBL was utilized to define the geometric pattern of top electrodes. Finally,
the top Au electrodes (50 nm) were defined by a final round of thermal evaporation to complete
the vertical EOR-based memristor devices.

Material and device characterizations
The morphology of the device was imaged with an optical microscope (Olympus BX51M) and a
scanning electron microscope (JEOL IT300). The surface roughness of the HfOₓ layer after the
ozone treatment was evaluated with an atomic force microscope (Park NX20). Raman
spectroscopy (Witec alpha300R) was performed to confirm the structural changes. X-ray
S
photoelectron spectroscopy (XPS) measurements were carriedS out using a PHI 5000 VersaProbe
III system (ULVAC-PHI, Inc.) with a monochromatic Al KEa source (1486.6 eV). High-resolution
R
spectra of Hf 4f, Mo 3d, and O 1s were collected, and all binding energies were calibrated using
P
the C 1s peak at 284.8 eV. A multibeam scanning e lectron microscopy–focused ion beam system
N
(JIB-4501, JEOL) was employed for cross-sectional sample preparation, with operation performed
I
using a 30 keV Ga⁺ ion beam. Specifically , a 1 μm-thick protective layer of platinum or carbon
E
was deposited in situ prior to ion-beamL irradiation. The target region was then milled using a 30 kV
C
accelerating voltage with a gallium ion beam current of 2.8-47 nA to obtain a thick lamella. The
I
lamella was subsequently liftTed out from the original substrate, mounted onto a STEM half-grid
R
using a micromanipulator, and further thinned to <100 nm using a 24-790 pA gallium ion beam to
A
prepare an electron-transparent specimen for STEM observation. All procedures were conducted
in a vacuum chamber maintained at ~10-4-10-5 Torr. For a detailed cross-sectional analysis, a
transmission electron microscopy (JEOL JEM-2100F TEM/scanning TEM instrument) operated
at 200 kV, equipped with EDS (Oxford INCA) mapping and EELS (Gatan Enfina), was used to
visualize the layer thickness, elemental distribution, and the presence of the oxygen-rich EOR layer.

Device electrical measurements
All electrical measurements were performed under ambient conditions except the temperature
measurement. The current-voltage (I-V) characteristics, endurance, and retention tests were
performed using a Lake Shore TTPX Probe Station with Keysight B1500A and Agilent B2912A
semiconductor parameter analyzers. Quasistatic direct current sweeps and dynamic electrical
pulses generated by a Keithley 4200 SCS were applied to investigate the memristive properties.
The switching speed of the device was evaluated using a pulse generator (Keithley 3402) and an
oscilloscope (GDS-1102B). The retention performance was measured at room temperature with a
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read voltage of 0.1 V. Temperature-dependent measurements were performed in a vacuum probe
station under a pressure of ~0.1 Pa at temperatures ranging from 77 K to 300 K.

Neural Network Simulation
A multilayer perceptron (MLP) model, consisting of 784 input neurons, 300 hidden neurons, and
10 output neurons, was implemented in Python. The experimentally obtained long-term
potentiation (LTP) and long-term depression (LTD) characteristics of the memristor were used to
update the synaptic weights in the network. The simulation was conducted using the Modified
National Institute of Standards and Technology (MNIST) dataset, and the recognition accuracy of
the network was calculated over 40 training epochs to evaluate the learning capability of the system.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, a 4.5 nm HfOx switching layer is confined between an HfS2 layer and a 3.5 nm elemental oxygen reservoir (EOR) layer. The device achieves ultrahigh Set/Reset switching speeds of 8 ns and 15 ns, respectively, with a normalized switching energy of 51 aJ. The HfOx tunneling layer above the HfS2 layer serves as a leakage barrier, enabling an ultralow leakage current of approximately 0.05 fA/μm², a large on/off ratio exceeding 1010, and long retention endurance up to 104 seconds. Furthermore, when modulated by pulse amplitude, the memristor demonstrates a high recognition accuracy of 97.0% in a convolutional neural network. Overall, this manuscript is well-organized and innovative. Considering that the overall quality, I recommend publication after a minor revision. Here are some suggestions for improvements:\n(1) The author can provide the electrical performance of the device when it is not oxidized? In Figure 2, the influence of ozone treatment time on the electrical performance of the fabricated EOR-based memristors is provided. What would the performance of devices without oxidation be like?\n(2) What role does the following MoS2 material play? To prevent the oxidation of MoS2, the author deliberately selected a larger HfS2 material. Why is HfS2 placed on top of MoS2 before oxidation? Oxidizing HfS2 first and then performing van der Waals assembly won't result in a flatter interface?\n(3) As shown in Figure 2, the performance of the device is good. How about the reproducibility? What is the difference between the EOR layer and the hafnium oxide layer? How can the EOR layer affect the performance of the fabricated memristor?\n(4) Figure 4 shows the comparison of the recognition accuracy of MNIST test images. Why are the starting points of the accuracy for the three comparisons different? How does the EOR-based memristor maintain desirable linearity?\n(5) It is not clear that how the ozone treatment is not affecting to MoS2? How the authors control the treatment condition considering each sample has different HfS2 thickness?\n(6) In particular, there is evidence for a clean vdW interface and EOR layer, which are novel insight relevant to a broad audience and to the best of my knowledge the first demonstration of the usage EOR layer to improve the memristor performance, details about the fabrication processes and the TEM measurements should be added in method section.\n(7) What are the parameters of the ozone treatment (i.e. chamber pressure, O2 partial pressure, UV light intensity, ...)? How can it be ensured that the topmost MoS2 layers are not oxidized, forming an MoO3 interlayer? Free monoatomic oxygen is a radical which readily bonds to any nearby molecule. Thus, there are certainly no oxygen atoms at the HfO2 surface, these are either O2 molecules or ozone (O3) molecules which are also highly reactive and thus likely bond to the HfO2 surface. Thus, the enhanced concentration corresponds to molecules of either O2, O3 or an off-stoichiometric phase of hafnium oxide with an increased O concentration at the amorphous HfO2 surface. Please modify the text in the main manuscript and the supplementary information accordingly.", "reviewer_2": "Reviewer Report\nThe authors report a van der Waals (vdW) heterostructure memristor that integrates an ultrathin HfOₓ switching layer and an “electroneutral elemental oxygen reservoir (EOR)” between MoS₂ and HfS₂. The device exhibits forming-free operation, an ultralow leakage current density of approximately 0.05 fA μm⁻², an on/off ratio exceeding 10¹⁰, and nanosecond-level switching speed (8 ns/15 ns) with a remarkably low switching energy of about 51 aJ. Such performance metrics are undoubtedly impressive and place the device among the best in the ultrathin or 2D-material-based memristor category. The manuscript is well-written, the experimental data are clean, and the proposed architecture is conceptually interesting. However, several central claims—particularly regarding the physical nature of the EOR layer and the mechanism of forming-free switching—are speculative and insufficiently supported by quantitative evidence. These gaps prevent the manuscript from reaching the level of mechanistic rigor expected in Nature Communications.\nThe main strength lies in its excellent device performance. The authors convincingly demonstrate that the vdW-derived EOR configuration can achieve extremely low leakage and high on/off ratio simultaneously, which has been a long-standing challenge in sub-5 nm oxide devices. The forming-free, ultrafast switching behavior and the low switching energy further highlight the potential of the approach for future low-power memory and neuromorphic computing. The device fabrication based on mild ozone oxidation of HfS₂ into HfOₓ is elegant and avoids plasma damage or interface disorder typical of ALD or sputtered films. TEM and EELS analyses provide a visually persuasive structural picture of the vertical stack, and temperature-dependent I–V measurements help delineate conduction regimes. Overall, the data presentation is of high quality and suggests that the authors have achieved careful experimental control.\nDespite these, reviewer suggests that the mechanistic interpretation requires much stronger evidence. The manuscript repeatedly refers to the presence of an “electroneutral elemental oxygen reservoir” at the MoS₂/HfOₓ interface, which is said to act as a tunneling barrier and diffusion regulator. However, the experimental basis for calling this layer “elemental” or “electroneutral” is insufficient. The EELS O–K edge data only reveal the presence of oxygen at the interface but do not identify its bonding state, valence, or neutrality. Without complementary depth-resolved XPS or ELNES analysis, it is impossible to confirm whether the layer consists of elemental oxygen, an oxygen-rich HfOᵧ, or MoOᵧ phase. This uncertainty is critical because the novelty and physical mechanism of the work hinge on the assumption that this layer is chemically distinct from conventional oxide reservoirs. At present, this interpretation remains speculative rather than conclusive.\nSimilarly, the proposed “dynamic equilibrium between oxygen and vacancies” as the origin of forming-free switching is presented as a central mechanistic claim but is not directly proven. The data show forming-free I–V characteristics and repeatability, but no kinetic or temperature-dependent experiments are performed to extract activation energies or diffusion coefficients that would substantiate such an equilibrium model. Without such analysis, the claim of a self-equilibrating oxygen-vacancy process remains qualitative and speculative. Quantitative studies of pulse-width, temperature, or interval dependence are needed to establish whether the switching indeed arises from a balanced oxygen exchange rather than conventional filamentary behavior limited by thickness or compliance.\nAnother point of concern is the assertion that the EOR suppresses crosstalk in array configurations. The manuscript presents no actual crossbar array data, sneak-path analysis, or half-select stress experiments. The claim is extrapolated from single-device leakage characteristics, which is not sufficient to demonstrate system-level selectivity or array compatibility. Similarly, while the manuscript attributes the ultralow leakage to the tunneling-barrier function of the EOR, temperature-dependent conduction analysis is limited to qualitative SCLC and Ohmic fitting. Quantitative extraction of barrier heights or Schottky/Poole–Frenkel/Fowler–Nordheim parameters would provide much stronger mechanistic grounding. Without these analyses, alternative explanations such as reduced trap density, series resistance in HfS₂, or interface depletion cannot be excluded.\nThe reliability data are also limited relative to the claims. The authors describe “robust retention and endurance,” yet only 10³ cycles and retention up to 10⁵ s (~28 h) at room temperature are presented. These values, though reasonable for a proof-of-concept, do not justify the use of “robust” or “stable” at the level expected for nonvolatile memory publications in Nature Communications. Measurements of endurance up to 10⁵–10⁶ cycles, retention at elevated temperatures (85–125 °C), and read-disturb or bias-stress testing are necessary to validate the long-term stability claims.\nThe comparison with the ALD-HfOₓ control device is helpful but not conclusive, as the two structures differ in fabrication method, oxidation time, and possibly thickness or interface roughness. Therefore, attributing the performance improvement solely to the EOR effect is premature. A head-to-head comparison with strictly identical geometries and electrode materials would be required to isolate the role of the EOR. Furthermore, the neuromorphic functionality—highlighted by a 97% MNIST recognition accuracy—is purely simulation-based. While the LTP/LTD data are experimentally measured, the neural network performance results from an offline simulation that does not include device-to-device variation, cycle-to-cycle drift, or retention degradation. Without hardware-in-the-loop or on-chip learning experiments, the neuromorphic claims should be presented more cautiously as “potential applications” rather than demonstrations.\nIn comparison to recent literature, the manuscript distinguishes itself by proposing a vdW-induced, neutral-oxygen interlayer rather than the conventional trilayer HfOₓ/HfO₂/HfOₓ or HfO₂/ZrO₂₋ₓ/HfO₂ stacks used as oxygen-vacancy reservoirs. Chemically, this approach could represent a new direction for interfacial control in 2D memristors. However, recent trilayer studies already demonstrate CMOS-compatible fabrication, array-level validation, and selector-free suppression of sneak paths—advantages that this work does not yet match. The novelty of a “neutral oxygen” reservoir is intriguing but must be proven chemically to stand as a clear advance over these prior works.\nIn conclusion, this manuscript presents a technically strong and potentially impactful concept with outstanding device performance. However, several mechanistic and interpretive claims are currently speculative or insufficiently evidenced. The work would benefit greatly from deeper chemical analysis of the EOR layer, quantitative conduction modeling, comprehensive reliability testing, and hardware-level neuromorphic validation. If these issues are addressed, the study could reach the broad and rigorous standard of Nature Communications. At its current stage, I recommend major revision. The authors are encouraged to provide quantitative and chemical evidence to solidify their mechanistic interpretation and to moderate claims that are presently speculative.", "reviewer_3": "This manuscript investigates HfS₂-based memristive devices. However, the work does not demonstrate substantial advances in either device performance or mechanistic understanding. Although the paper claims a sub-5-nm switching region, this refers merely to the film thickness rather than the actual device footprint. I initially expected lateral device scaling, but the reported vertical dimension does not provide meaningful novelty.\nThe manuscript further claims “high-speed” switching, including the 8 ns / 15 ns switching times emphasized in the title and abstract. In reality, the devices are driven by 100 ns pulses, with the current merely reaching a steady level at 8 ns / 15 ns. This does not constitute stable switching. In the community, a switching speed is typically accepted only if the device can reliably toggle under pulses of that duration, and the subsequent voltage/current holding sequence confirms stable filament formation or rupture. The same issue applies to the purported “ultralow energy” operation, where only the portion of the current transient up to stabilization is considered—this is not an appropriate metric. The MNIST recognition demonstration based on simulations also does not introduce meaningful novelty.\nAdditional concerns:\n1. Device area: The manuscript does not specify the device area, which is critical for evaluating performance. Only current density is reported, without the absolute current values.\n2. Endurance testing: Endurance is typically evaluated using ~200 µs pulses. What would the endurance look like under 8 ns / 15 ns pulses? The reported endurance of ~1,000 cycles is insufficient for practical applications; can it be improved?\n3. Retention testing: Retention should not be evaluated only at room temperature. Measurements at 85 °C—or preferably 125 °C—are standard practice.\n4. Memory window: The experimental results indicate a memory window of approximately 3–5 orders of magnitude, with noticeable variation across tests. This is far from the 10-order-of-magnitude window claimed in the manuscript.\n5. Inconsistencies in resistance values: The reported high-/low-resistance states vary significantly across figures. In Fig. 2b they are 10⁻¹² / 10⁻¹⁶ A, in Fig. 3h they become 10⁻¹³ / 10⁻¹⁶ A, while in Fig. S3 they are around 10⁻¹⁶ / 10⁻⁷ A. These inconsistencies need to be clarified and justified."}, "author_response": "We sincerely thank the reviewers for the valuable comments, which have\nsignificantly improved the manuscript. Please find our point-by-point response to\nthe comments below.\nResponse to Reviewer #1:\nIn this work, a 4.5 nm HfO switching layer is confined between an HfS layer and a\nx 2\n3.5 nm elemental oxygen reservoir (EOR) layer. The device achieves ultrahigh\nSet/Reset switching speeds of 8 ns and 15 ns, respectively, with a normalized switching\nenergy of 51 aJ. enabling an ultralow leakage current of approximately 0.05 fA/μm2, a\nlarge on/off ratio exceeding 1010, and long retention endurance up to 104 seconds.\nFurthermore, when modulated by pulse amplitude, the memristor demonstrates a high\nrecognition accuracy of 97.0% in a convolutional neural network. Overall, this\nmanuscript is well-organized and innovative. Considering that the overall quality, I\nrecommend publication after a minor revision. Here are some suggestions for\nimprovements:\nAnswer: We sincerely thank the reviewer for thoroughly examining our manuscript and\nfor their valuable comments. We appreciate the recognition of the device structure,\nswitching performance, ultralow leakage characteristics, and its potential for\nneuromorphic applications. We have carefully considered all the constructive\nsuggestions provided and have revised the manuscript accordingly. Detailed point-by-\npoint responses to each comment are included below. We believe that these revisions\nhave further improved the clarity and quality of the manuscript.\n1. The author can provide the electrical performance of the device when it is not\noxidized? In Fig. 2, the influence of ozone treatment time on the electrical performance\nof the fabricated EOR-based memristors is provided. What would the performance of\ndevices without oxidation be like?\nAnswer: We thank the reviewer for this valuable suggestion. We fabricated control\ndevices without ozone treatment, as shown in Fig. R1a. The corresponding I-V curves\n(Fig. R1b) exhibit negligible resistive switching behavior. In our device design, the\nswitching HfO layer is sandwiched between two 2D material layers (HfS and MoS ).\nx 2 2\nInstead, the formation of conductive filaments driven by V is the fundamental\nO\noperating mechanism of EOR memristors. However, the control MoS /HfS stack was\n2 2\nfabricated via mechanical exfoliation in a nitrogen-filled glove box, resulting in a very\nlow density of vacancies and trapped charges. Without the ozone treatment to generate\nsufficient V , the device lacks the necessary conditions for filament formation, and the\nO\nabsence of the HfO tunneling barrier leads to low resistance. Consequently, no\nx\neffective memristive window is observed. We have included these results and the\ncorresponding discussion in the revised manuscript and Supplementary Information.\n2 2\nOptical microscopy image. (b) I-V curves showing negligible resistive switching behavior.\n2. What role does the following MoS material play? To prevent the oxidation of MoS ,\n2 2\nthe author deliberately selected a larger HfS material. Why is HfS placed on top of\n2 2\nMoS before oxidation? Oxidizing HfS first and then performing van der Waals\n2 2\nassembly won't result in a flatter interface?\nAnswer: We thank the reviewer for this valuable comment. First, the atomically flat\nsurface of MoS serves as an ideal template for the growth of the oxide switching layer.\nultrathin films. Furthermore, this high-quality interface reduces trap states and\nsuppresses unwanted charge trapping, which stabilizes the local electric field\ndistribution and improves the uniformity of the switching process. The EOR layer\noriginates from the vdW interface between HfS and MoS , where the oxygen-rich\n2 2\nenvironment promotes oxygen accumulation. Crucially, oxygen migration across the\nMoS layer is energetically forbidden due to a large energy barrier (> 6 eV), and a high\n2\nenergy barrier of 1.21 eV for oxygen atoms to traverse from the HfO surface to the\nx\nMoS interface. Consequently, oxygen is confined between the HfO and MoS , which\n2 x 2\neffectively suppresses leakage paths in the high-resistance state (HRS).\nRegarding the fabrication process, the HfS flake is designed to fully overlap the\n2\ntreatment and ensures the confinement of the EOR layer. In contrast, if the HfS were\n2\noxidized prior to transfer, the resulting HfO surface would be directly exposed during\nx\nvan der Waals assembly. This surface is highly hydrophilic and prone to contamination\nby adsorbates. Additionally, since the oxidation of HfS is not a self-limiting process, it\n2\nleads to significant thickness variations in the HfO . This roughness is detrimental to\nx\ncontrolling the uniformity of sub-10 nm films.\n3. As shown in Fig. 2, the performance of the device is good. How about the\nreproducibility? What is the difference between the EOR layer and the hafnium oxide\nlayer? How can the EOR layer affect the performance of the fabricated memristor?\nreproducibility and the role of the EOR layer. To systematically evaluate reproducibility,\nwe fabricated 100 devices across four 5 × 5 arrays. The statistical distributions of key\nperformance indicators, including off-state current, on-off ratio, and switching voltages,\nare presented in Fig. R2. These results demonstrate excellent device-to-device\nuniformity. Mechanistically, the EOR layer confines the HfO switching layer within a\nx\nuniform structure and ensures a homogeneous electric field distribution, which is\nessential for reliable operation. Moreover, the EOR layer forms a pristine interface with\nthe ultrathin oxide layer. It functions as a tunneling barrier to suppress the stochastic\nmigration of V as well as leakage current. Consequently, the integration of the EOR\nO\nlayer with the switching oxide enables highly stable memristor operation with ultralow\npower consumption. We have added additional data and a detailed discussion in the\nrevised manuscript.\nFig. R2 | Statistical electrical performance of 100 EOR-based memristors. (a) Superimposed I-V\ncurves. (b) Statistical distributions of currents in the HRS and LRS. (c) On/off ratio distributions.\n(d) SET and RESET voltage distributions.\n4. Fig. 4 shows the comparison of the recognition accuracy of MNIST test images. Why\nare the starting points of the accuracy for the three comparisons different? How does\nthe EOR-based memristor maintain desirable linearity?\nAnswer: We thank the reviewer for this valuable comment. Different devices exhibit\nvarying degrees of initial conductance uniformity. In ideal memristors, the standard\ndeviation of initial conductance is negligible, allowing initialized weights to closely\nmatch the target values, thus yielding higher initial accuracy. In contrast, conventional\nmemristors often show larger variations due to the stochastic distribution of oxygen\n3\nvacancies. This causes initialized weights to deviate from the ideal values, consequently\ndegrading the initial accuracy. In our proposed device, both the thickness of the HfO\nx\nswitching layer and the oxygen concentration in the EOR layer are highly uniform. The\nEOR-based memristor exhibits stable analog switching characteristics. As a result, the\nconductance modulation is highly linear with respect to the applied programming\ncurrent. Moreover, during repeated weight updates, the EOR layer acts as a reservoir to\ncontinuously replenish the oxygen atoms consumed in the HfO layer. Therefore, the\nx\nabundant oxygen supply in the EOR layer facilitates precise control over the evolution\nof conductive filaments. This enhances the synaptic linearity, thereby increasing the\nrecognition accuracy of the device.\n5. It is not clear that how the ozone treatment is not affecting to MoS ? How the authors\n2\ncontrol the treatment condition considering each sample has different HfS thickness?\ninvolves reactive species that could modify the surface, we designed the structure to\nprevent damage. Specifically, the HfS flake fully overlaps the MoS channel and\n2 2\ncontact regions to suppress inadvertently introduced defects and shield the MoS from\n2\nultraviolet (UV) illumination, as indicated in Fig. R3a. Fig. R3b shows the Raman\nspectra from different regions of the device. The attenuation of the HfS E /A phonon\n2 g 1g\nmodes confirms the complete transition to amorphous HfO . Additionally, the slight\nx\nshift in the MoS E1 /A peaks is attributed to the doping effect induced by the\n2 2g 1g\nadjacent EOR layer. Our calculations indicate a high energy barrier of 1.21 eV for\noxygen atoms to traverse from the HfO surface to the MoS interface. Consequently,\nx 2\nthe accumulated oxygen tends to adsorb at the HfO interface, as shown in Fig. R3c. In\nx\nour experiments, HfS flakes with a thickness of 8-12 layers were selected to generate\n2\nthe HfO layer. By precisely controlling the ozone treatment duration, we can obtain an\nx\nultrathin oxide layer while preserving a residual thickness of HfS . Since the device is\n2\na vertically stacked structure and the residual HfS retains its single-crystal nature with\n2\nexcellent conductivity, slight thickness fluctuations have a negligible impact on the\nvertical current transport. Crucially, the HRS current and switching behavior are\ndominated by the thickness of the HfO switching layer, which is effectively tuned by\nx\nthe ozone treatment time.\nFig. R3 | (a) Optical microscopy image of a 5 × 5 memristor array. (b) Raman spectra from different\n4\nregions of the device. (c) Oxygen migration energy barrier calculated using the nudged elastic band\n(NEB) method.\n6. In particular, there is evidence for a clean vdW interface and EOR layer, which are\nnovel insight relevant to a broad audience and to the best of my knowledge the first\ndemonstration of the usage EOR layer to improve the memristor performance, details\nabout the fabrication processes and the TEM measurements should be added in method\nsection.\nAnswer: We thank the reviewer for this valuable comment. The evidence for the EOR\nlayer and the vdW interface is indeed a key strength of this work. Regarding the\npreparation of TEM samples: Briefly, an HfS flake was placed onto the MoS via a\n2 2\nphysical transfer method. Ozone treatment was then introduced to oxidize the HfS into\n2\nan HfO /HfS /HfO stack. Finally, a 50 nm Au film was deposited via thermal\nx 2 x\nevaporation. The TEM sample preparation was performed using a JIB-4501 focused\nion beam system, utilizing a liquid gallium (Ga) ion source. The procedure was as\nfollows: First, the sample was securely mounted, and the region of interest (ROI) was\nlocated and aligned using the SEM mode. Coarse milling was then performed with a\nrelatively high ion beam current to remove excess material and shape the lamella.\nSubsequently, the beam current was gradually reduced for fine milling. To achieve a\nhigh-quality surface finish suitable for atomic-resolution analysis, a final polishing step\nwas conducted using a low-current ion beam. This step minimized surface damage and\nimproved the clarity of the cross-section. We have updated the Methods section with\nthese experimental details. Additionally, we have included detailed parameters for the\nTEM sample preparation and EELS/EDS mapping in the revised manuscript.\nFig. R4 | Cross-sectional cross profile of the TEM sample.\n7. What are the parameters of the ozone treatment (i.e. chamber pressure, O partial\n2\npressure, UV light intensity, ...)? How can it be ensured that the topmost MoS layers\n2\nare not oxidized, forming an MoO interlayer? Free monoatomic oxygen is a radical\n3\nwhich readily bonds to any nearby molecule. Thus, there are certainly no oxygen atoms\nat the HfO surface, these are either O molecules or ozone (O ) molecules which are\n2 2 3\nalso highly reactive and thus likely bond to the HfO surface. Thus, the enhanced\n2\nconcentration corresponds to molecules of either O , O or an off-stoichiometric phase\n2 3\nof hafnium oxide with an increased O concentration at the amorphous HfO surface.\n5\naccordingly.\nAnswer: Ozone was generated via ultraviolet irradiation under ambient conditions\nusing a 300 W UV lamp. The UV intensity was maintained at 50 mW/cm², resulting in\na mild substrate temperature of 310 K. To prevent potential degradation of the\nunderlying MoS during this process, the HfS flake was designed to fully overlap the\n2 2\nMoS channel and the contact regions.\nmemristors to analyze the bonding states of oxygen. As illustrated in Fig. R5a,\nconsidering the XPS sampling depth of approximately 5 nm, the sulfur signal shows a\nsignificant increase after 2 min of etching, suggesting that the signal obtained at roughly\n3 min corresponds to the HfO switching layer. When the etching time reaches 5 min,\nx\nthe Mo signal rises sharply, suggesting that the XPS spectra obtained at 6 minutes\nprimarily originate from the EOR layer. Our analysis of the oxygen bonding states\nreveals that on the device surface (Fig. R5b), the content of high binding energy oxygen\nis relatively high, likely due to surface-adsorbed oxygen. In the HfO switching layer\nx\n(Fig. R5c), the concentration of V increases while the high binding energy oxygen\nO\ncomponent decreases. Notably, the signal corresponding to the EOR layer is dominated\nby high binding energy components, while signals associated with V and Hf-O bonds\nO\ndiminish significantly. Combined with the EELS results, we attribute this layer\nprimarily to O-O bonds (Fig. R5d), indicating the presence of molecular oxygen (O ).\n2\n(O -, bond order = 1.5) are lower than that of neutral oxygen (O , bond order = 2).\n2 2\nConsequently, ionic oxygen species are metastable and tend to react to achieve the\nstable O electronic configuration; thus, we propose that the EOR layer is composed\n2\nprimarily of electroneutral oxygen. Additionally, due to the high V concentration in\nO\nthe HfO switching layer adjacent to the HfS flake, a metallic hafnium phase is\nx 2\nobserved (Fig. R5e), which likely facilitates the forming-free operation of the device.\nFurthermore, as shown in Fig. R5f, after 6 min of etching, negligible Mo-O bond-\nrelated species are observed, confirming that no MoO interlayer is formed. We have\n3\nincluded this detailed analysis in the revised manuscript.\nof etching time. (b–d) Depth-resolved O 1s XPS spectra obtained at (b) the surface, (c) 3 min, and\n(d) 6 min of etching. (e) Hf 4f spectrum after 3 min. (f) Mo 3d spectrum after 6 min.\nThe authors report a van der Waals (vdW) heterostructure memristor that integrates an\nultrathin HfO switching layer and an “electroneutral elemental oxygen reservoir\nx\n(EOR)” between MoS and HfS . The device exhibits forming-free operation, an\n2 2\nultralow leakage current density of approximately 0.05 fA μm-2, an on/off ratio\nexceeding 1010, and nanosecond-level switching speed (8 ns/15 ns) with a remarkably\nlow switching energy of about 51 aJ. Such performance metrics are undoubtedly\nimpressive and place the device among the best in the ultrathin or 2D-material-based\nmemristor category. The manuscript is well-written, the experimental data are clean,\nand the proposed architecture is conceptually interesting. However, several central\nclaims—particularly regarding the physical nature of the EOR layer and the mechanism\nof forming-free switching—are speculative and insufficiently supported by quantitative\nevidence. These gaps prevent the manuscript from reaching the level of mechanistic\nrigor expected in Nature Communications.\nAnswer: We would like to express our sincere gratitude to the reviewer for their\nthroughout examination of our manuscript and for their valuable insights. We are\nparticularly encouraged by the recognition of the device performance demonstrated in\nthis study. Following the reviewer's suggestions, we have carefully addressed all\ncomments to better highlight the innovation and novelty of our work, and have revised\nthe manuscript accordingly.\nIn response to the concern regarding the physical nature of the EOR layer, we\nconducted additional depth-resolved XPS measurements and ELNES analysis to clarify\nthe chemical bonding state of oxygen and substantiate our claims. A high energy barrier\nof 1.21 eV is required for an oxygen atom to traverse from the HfO surface to the MoS\nx 2\ninterface, as calculated by the nudged elastic band (NEB) method. Consequently, the\naccumulated oxygen tends to adsorb onto the HfO switching layer. Moreover, the XPS\nx\nsignals from the EOR layer primarily exhibit peaks at high binding energies, while the\nsignals for V and Hf-O bonds are significantly reduced. Considering the EELS results,\nO\nwe attribute this layer primarily to O-O bonds. This supports our claim that the EOR\nlayer serves as an oxygen reservoir, thereby facilitating the rupture of conductive\nfilaments in the high-resistance state (HRS).\nIn response to the concern regarding the forming-free switching mechanism, this\nbehavior is attributed to the dynamic equilibrium of V facilitated by the EOR layer.\nO\nBy fitting the data to the Arrhenius equation, we estimated the activation energy for V\nO\nmigration to be 39.2 meV. Since the accumulated oxygen tends to adsorb onto the HfO\nx\nswitching layer, the V near the EOR layer undergo recombination, potentially\nO\nrendering the effective thickness of the HfO switching layer thinner than its physical\nx\nthickness (~4.5 nm). Furthermore, XPS results indicate abundant V and metallic\nO\nhafnium within the HfO layer, leading to the partial formation of conductive paths.\nx\nThis pre-existing state reduces the operating voltage and facilitates forming-free\nbehavior.\n8\n1. The main strength lies in its excellent device performance. The authors convincingly\ndemonstrate that the vdW-derived EOR configuration can achieve extremely low\nleakage and high on/off ratio simultaneously, which has been a long-standing challenge\nin sub-5 nm oxide devices. The forming-free, ultrafast switching behavior and the low\nswitching energy further highlight the potential of the approach for future low-power\nmemory and neuromorphic computing. The device fabrication based on mild ozone\noxidation of HfS into HfO is elegant and avoids plasma damage or interface disorder\n2 x\ntypical of ALD or sputtered films. TEM and EELS analyses provide a visually\npersuasive structural picture of the vertical stack, and temperature-dependent I–V\nmeasurements help delineate conduction regimes. Overall, the data presentation is of\nhigh quality and suggests that the authors have achieved careful experimental control.\nAnswer: We sincerely thank the reviewer for the thorough assessment of our work and\nfor recognizing the strengths of our device performance, data quality, and fabrication\nstrategy. The detailed comments provided instrumental guidance for refining the\nmanuscript and deepening the discussion of the underlying mechanisms. In response,\nwe have carefully addressed these concerns through additional analyses and revised the\nmanuscript accordingly.\n2. Despite these, reviewer suggests that the mechanistic interpretation requires much\nstronger evidence. The manuscript repeatedly refers to the presence of an\n“electroneutral elemental oxygen reservoir” at the MoS /HfO interface, which is said\n2 x\nto act as a tunneling barrier and diffusion regulator. However, the experimental basis\nfor calling this layer “elemental” or “electroneutral” is insufficient. The EELS O–K\nedge data only reveal the presence of oxygen at the interface but do not identify its\nbonding state, valence, or neutrality. Without complementary depth-resolved XPS or\nELNES analysis, it is impossible to confirm whether the layer consists of elemental\noxygen, an oxygen-rich HfOᵧ, or MoOᵧ phase. This uncertainty is critical because the\nnovelty and physical mechanism of the work hinge on the assumption that this layer is\nchemically distinct from conventional oxide reservoirs. At present, this interpretation\nremains speculative rather than conclusive.\nAnswer: We thank the reviewer for this important and constructive comment. In\nresponse to this concern, we have conducted depth-resolved XPS measurements and\nELNES analysis to clarify the bonding states of oxygen in the memristor stack. As\nshown in Fig. R1a, given that the XPS sampling depth is approximately 5 nm, a\nsignificant increase in the S signal is observed after 2 min of etching. Therefore, the\nsignal obtained at approximately 2 min corresponds to the HfS layer. At an etching\n2\ntime of 3 min, the hafnium content begins to decrease while the oxygen concentration\nincreases, identifying the HfO switching layer. After approximately 6 min of etching,\nx\nthe Mo content increases significantly, suggesting that the measurement region has\nprobed into the EOR layer.\nWe analyzed the oxygen bonding states across different layers. On the device\nsurface (Fig. R1b), the proportion of oxygen species at high binding energies is\n9\nrelatively high (42%), which is likely ascribed to surface-adsorbed oxygen. In the HfO\nx\nswitching layer (Fig. R1c), the V concentration rises to 31%, while the high-binding-\nO\nenergy oxygen component (assigned to O-O bonds) decreases to 26%. Notably, the\nsignal originating from the EOR layer (Fig. R1d) is predominately composed of the\nhigh-binding-energy component (reaching 52%), while the signals for V and Hf-O\nO\nbonds decrease significantly. Therefore, in conjunction with the EELS results, we\nattribute this layer primarily to O-O bonds. In addition, bond order correlates with bond\nstability. A higher bond order implies greater bond energy and stability. The bond order\nof peroxide (O 2-) is 1, and that of superoxide (O -) is 1.5, both of which are lower than\n2 2\nthat of neutral oxygen (O , bond order = 2). Lower bond orders lead to reduced bond\n2\nenergies and thermodynamic instability. Consequently, O 2- and O - species are\n2 2\nmetastable and highly reactive, tending to gain or lose electrons to achieve the more\nstable O electronic configuration. Thus, based on the evidence of O-O bonding, we\n2\npropose that the EOR layer is composed primarily of neutral oxygen. Furthermore,\ngiven the relatively high concentration of V in the HfO switching layer adjacent to\nO x\nthe HfS , a metallic hafnium phase is\n2\nobserved (Fig. R1e), which may facilitate the forming-free operation of the EOR-based\nmemristor. As shown in Fig. R1f, negligible\nsignals related to Mo-O bonds are detected after 6 min of etching, indicating that no\nMoO interlayer is formed.\nDepth-resolved O 1s XPS spectra measured (b) at the surface, and after (c) 3 min and (d) 6 min of\netching. (e) Hf 4f XPS spectrum obtained after 3 min of etching. (f) Mo 3d XPS spectrum acquired\nafter 6 min of etching.\nThe oxygen migration energy barrier was also calculated using the nudged elastic\nband (NEB) method, as shown in Fig. R2. A high energy barrier of 1.21 eV is required\n10\nfor an oxygen atom to traverse from the HfO surface to the MoS interface.\nx 2\nConsequently, the accumulated oxygen tends to adsorb onto the HfO surface. This also\nx\nexplains the formation of the oxygen-rich HfO layer and the absence of Mo-oxide\nx\nspecies. The O K-edge ELNES spectra acquired from the HfO switching layer and the\nx\nEOR layer are shown in Fig. R2b. The O K-edge of the HfO is dominated by two peaks,\nx\nlabeled p1 and p2, centered at 534.5 eV and 538.1 eV, respectively. The splitting\nbetween these two peaks is approximately 3.6 eV. Since the peak structure is sensitive\nto the presence of oxygen-related defects, the peaks appear broadened and less well-\nresolved [1]. As previously reported for transition metal oxide films with excess oxygen,\nthe p1–p2 peak splitting is typically absent; instead, the O K-edge of the EOR layer\nexhibits a distinct single peak at 538.3 eV [2–4]. Therefore, these results further confirm\nthe presence of an oxygen-rich HfO layer and the EOR layer within the stack. We have\nx\nadded the corresponding description to the revised manuscript.\nFig. R2 | (a) Potential energy profile for oxygen migration calculated using the nudged elastic band\n(NEB) method. (b) O K-edge ELNES spectra acquired from the HfO switching layer and the EOR\nx\nlayer.\n3. Similarly, the proposed “dynamic equilibrium between oxygen and vacancies” as the\norigin of forming-free switching is presented as a central mechanistic claim but is not\ndirectly proven. The data show forming-free I–V characteristics and repeatability, but\nno kinetic or temperature-dependent experiments are performed to extract activation\nenergies or diffusion coefficients that would substantiate such an equilibrium model.\nWithout such analysis, the claim of a self-equilibrating oxygen-vacancy process\nremains qualitative and speculative. Quantitative studies of pulse-width, temperature,\nor interval dependence are needed to establish whether the switching indeed arises from\na balanced oxygen exchange rather than conventional filamentary behavior limited by\nthickness or compliance.\nAnswer: We thank the reviewer for this important and constructive comment.\nTemperature-dependent electrical measurements were conducted over the range of 77\nK to 300 K to investigate the thermally activated transport of V . The activation energy\nO\nwas determined by fitting the current data to the Arrhenius equation: I = I × exp(-\nA 0\nconstant, and T is the absolute temperature. The extracted activation energy was 39.2\nmeV.\nvoltage. Extrapolation to zero bias yields Q , from which an activation energy of 39.2 meV is\n0\ncalculated.\nTo investigate the kinetics of forming-free switching, pulse-width-dependent\nmeasurements were performed by varying the pulse duration from 10 ns to 100 ns, as\nshown in Fig. R4. The results demonstrate that the current increases with increasing\npulse width.\nFig. R4 | Read current of the EOR-based memristor measured at different pulse widths.\nIn our EOR-based memristor, the migration of V under an electric field drives\nO\nthe formation of conductive filaments (CFs). The EOR layer is strategically positioned\nbetween the 4.5 nm HfO switching layer and the MoS , creating a nanoscale confined\nx 2\nenvironment. This architecture effectively inhibits the stochastic diffusion of V ,\nO\nrendering the formation and rupture of CFs more deterministic, thereby facilitating\nstable forming-free switching. Moreover, the EOR layer at the HfO /MoS interface\nx 2\nleads to a locally modulated V concentration. This region serves as a functional\nO\noxygen reservoir that promotes the recombination of V with oxygen species during\nO\nthe reset process, resulting in an enhanced ON/OFF ratio and robust endurance.\nThis dynamic equilibrium model aligns with mechanisms reported in prior studies, where\nforming-free characteristics are achieved by engineering an oxygen-rich interface to\nsuppress the HRS current. Therefore, we adopt this model to explain the constrained\nfilament growth, which accounts for the low-power LRS observed in our EOR-based\nmemristors.\n4. Another point of concern is the assertion that the EOR suppresses crosstalk in array\nconfigurations. The manuscript presents no actual crossbar array data, sneak-path\nanalysis, or half-select stress experiments. The claim is extrapolated from single-device\n12\nleakage characteristics, which is not sufficient to demonstrate system-level selectivity\nor array compatibility. Similarly, while the manuscript attributes the ultralow leakage\nto the tunneling-barrier function of the EOR, temperature-dependent conduction\nanalysis is limited to qualitative SCLC and Ohmic fitting. Quantitative extraction of\nbarrier heights or Schottky/Poole–Frenkel/Fowler–Nordheim parameters would\nprovide much stronger mechanistic grounding. Without these analyses, alternative\nexplanations such as reduced trap density, series resistance in HfS , or interface\n2\ndepletion cannot be excluded.\nAnswer: We appreciate the valuable suggestions. Accordingly, we conducted a\nsystematic investigation of the memristive behavior in 5 × 5 crossbar arrays (Fig. R5).\nRepresentative bipolar I-V characteristics were obtained from 100 EOR-based\nmemristors integrated into four separate 5 × 5 arrays. The highly consistent electrical\nperformance indicates that the integrated HfO /EOR heterostructure effectively\nx\nsuppresses sneak-path currents, thereby mitigating crosstalk within the crossbar\narchitecture. As suggested by the reviewer, a sneak-path analysis is provided in Fig.\nR5b. In conventional arrays without adequate leakage control, the programming of a\nselected cell can inadvertently disturb half-selected and unselected cells. The adopted\nvoltage pulsing scheme involves applying the programming voltage to the selected cell,\nwhile half-selected cells are biased at a fractional voltage. Consequently, only the\nselected cell undergoes conductance switching upon the application of pulses, while the\nunselected cells in the array remain unperturbed (Fig. R5c).\nFig. R5 | Sneak-path analysis of the memristor arrays. (a) Optical image of a 5 × 5 memristor array.\n(b) Schematic illustration showing the definitions of different device states within the array. V\nprog\ndenotes the programming voltage applied to the word lines (WLs) to induce resistive switching in\nthe selected cell. (c) Conductance evolution of selected, half-selected, and unselected cells in the\narray during pulse programming (±1.5 V, 20 ns).\nIn addition, specific cells within the array were programmed into the high-\nresistance state (HRS) to form predefined patterns (Fig. R6). The read currents of all\ncells were measured at a read voltage of 0.1 V and mapped onto a conductance heatmap.\nIn this map, the programmed HRS cells are manifested as dark regions, corresponding\nto significantly reduced conductance, while the unprogrammed cells remain in the low-\nresistance state (LRS), appearing as light regions with high conductance. The distinct\ncontrast between programmed and unprogrammed cells, along with the accurate\n13\nreproduction of the target patterns, demonstrates that the readout process is not\ncompromised by sneak currents. This result confirms that crosstalk within the proposed\nmemristor array is effectively suppressed, allowing each cell to be individually\naddressed and accessed without interference from adjacent devices.\nFig. R6 | Demonstration of digital pattern storage in a 5 × 5 crossbar memory array. The array\nexhibits programmed resistance states (gray for HRS and white for LRS). The designed pattern was\nwritten into the array using pulse programming. The x and y axes denote device coordinates (row\nand column indices), while the color scale (z -axis) represents the measured conductance values of\nthe memory cells.\nThe extracted Schottky barrier height is 15.4 meV, as shown in Fig. R7.\nConsequently, this reduced barrier height lowers the energy threshold required for the\nformation of conductive filaments. Under an electric field, V are more susceptible to\nO\nmigration within the HfO switching layer. Furthermore, the presence of metallic Hf\nx\nwas detected in the XPS spectra of the HfO layer adjacent to the HfS flake. This\nx 2\nmetallic component is a critical factor in realizing such a low interfacial barrier.\nCollectively, these factors enable the EOR-based memristor to exhibit forming-free\n14\nswitching with minimal power consumption.\nFig. R7 | Schottky barrier height extraction for the EOR-based memristor. (a) Arrhenius plot\nshowing the dependence of IT-2 on inverse temperature T-1, where the solid lines represent linear\nfits. (b) Dependence of the extracted slope S on the bias voltage. Extrapolation to zero bias yields\nS , which is utilized to calculate the effective Schottky barrier height.\n0\n5. The reliability data are also limited relative to the claims. The authors describe\n“robust retention and endurance,” yet only 103 cycles and retention up to 105 s (~28 h)\nat room temperature are presented. These values, though reasonable for a proof-of-\nconcept, do not justify the use of “robust” or “stable” at the level expected for\nnonvolatile memory publications in Nature Communications. Measurements of\nendurance up to 105–106 cycles, retention at elevated temperatures (85–125 °C),\nand read-disturb or bias-stress testing are necessary to validate the long-term stability claims.\nAnswer: We appreciate the valuable suggestions. We agree that retention at elevated\ntemperatures and extended endurance testing are crucial for a rigorous assessment of\nnonvolatile memory reliability. While we initially reported endurance data up to 103\ncycles as a proof-of-concept, we have now extended the testing to 105 cycles, as shown\nin Fig. R8a. These additional measurements demonstrate that the device exhibits\nexcellent endurance without significant performance degradation. A stable ON/OFF\nratio and small fluctuation in switching characteristics was observed throughout the\ncycling process. These results strongly support our claim of \"robust endurance\" and\nconfirm that the device is capable of prolonged operation, a key requirement for\nnonvolatile memory applications. We have also addressed the concern regarding data\nretention by conducting tests at elevated temperatures. Specifically, retention\ncharacteristics were measured at 85 °C and 125 °C, as shown in Fig. R8b and R8c. The\nresults demonstrate that the device retains its memory states for over 105 s, even at these\nelevated temperatures. This retention performance is consistent with the behavior\nobserved at room temperature and validates the device's thermal stability—a critical\nrequirement for practical applications in harsh environments.\ncharacteristics monitored over 105 cycles using bipolar voltage pulses of ±1.5 V (pulse width: 200\nµs). (b, c) Retention characteristics measured at 85 °C and 125 °C, respectively.\n6. The comparison with the ALD-HfO control device is helpful but not conclusive, as\nx\nthe two structures differ in structures differ in fabrication method, oxidation time, and possibly thickness\nor interface roughness. Therefore, attributing the performance improvement solely to\nthe EOR effect is premature. A head-to-head comparison with strictly identical\ngeometries and electrode materials would be required to isolate the role of the EOR.\nFurthermore, the neuromorphic functionality—highlighted by a 97% MNIST\nrecognition accuracy—is purely simulation-based. While the LTP/LTD data are\nexperimentally measured, the neural network performance results from an offline\nsimulation that does not include device-to-device variation, cycle-to-cycle drift, or\nretention degradation. Without hardware-in-the-loop or on-chip learning experiments,\nthe neuromorphic claims should be presented more cautiously as “potential applications”\nrather than demonstrations.\nAnswer: We appreciate the valuable suggestions. We agree with the reviewer that the\nALD- HfO control device differs significantly from our EOR-based memristor in terms\nx\nof fabrication methodology, oxidation duration, thickness, and interface roughness.\nConsequently, a direct comparison with the ALD-based device may lack sufficient rigor\nwithin the context of this study. Accordingly, we have removed these data and the\ncorresponding descriptions from the revised manuscript.\nFigure 9 illustrates the thickness evolution of HfS during its conversion to HfO\n2 x\nvia ozone oxidation. With increasing ozone treatment time, the HfS flake gradually\n2\ntransforms from a partially oxidized state to a completely converted HfO layer. Based\nx\non the initial thickness of HfS (11.17 nm, Fig. R9b) and the final thickness of the fully\n2\noxidized sample (15.13 nm, Fig. R9i) measured by atomic force microscopy, the\nexpansion ratio was determined to be 1.35. To quantify the thickness of the converted\nHfO at various stages, we defined two parameters: the total AFM-measured thickness\nx\n(t) and the thickness of the residual unconverted HfS (x). Their relationship is\n2\nexpressed by the following equation: t=x+1.35×(11.17-x). By determining the value of\nx, the thickness of the formed HfO layer can be precisely derived. This indicates that\nx\nincreasing the ozone treatment time can increase the thickness of the HfO layer.\nx\nx x 2 x\nthe HfO thickness as a function of ozone treatment time. (b-i) Evolution of height profiles with\nx\nvarying ozone treatment time. Each inset shows an AFM image. Scale bar: 2 μm.\nTo elucidate the influence of switching layer thickness and the EOR layer on\ndevice performance, a direct, systematic comparison using devices with identical\ngeometries and electrode materials is provided in Fig. R10. Since the oxidation of HfS\n2\ninto HfO is not a self-limiting process, prolonging the oxidation time (ozone treatment\nx\nduration) results in an increased HfO thickness. As the oxidation duration exceeds 40\nx\nmin, the high-resistance state (HRS) current exhibits a continuous rise. This is primarily\nattributed to the enhanced presence of remnant conductive filaments within the thicker\nswitching layer, which serve as leakage paths and lead to a higher HRS current.\nMoreover, the magnitudes of V and V increase, as a thicker switching layer\nSET RESET\nrequires a higher electric field to induce resistive switching. Therefore, reducing\nthe thickness of the switching layer is effective in lowering the operating voltages.\nSimultaneously, the introduction of the EOR layer facilitates the attainment of an\nextremely low HRS current.\nx\ntreatment duration on the electrical characteristics of the EOR-based memristor. (b) Dependence of\nLRS and HRS currents on oxidation duration. (c) Dependence of switching voltages (V and\nSET\nV ) on oxidation duration.\nRESET\n7. In comparison to recent literature, the manuscript distinguishes itself by proposing a\nvdW-induced, neutral-oxygen interlayer rather than the conventional trilayer\nHfO /HfO /HfO or HfO /ZrO /HfO stacks used as oxygen-vacancy reservoirs.\nx 2 x 2 2-x 2\nChemically, this approach could represent a new direction for interfacial control in 2D\nmemristors. However, recent trilayer studies already demonstrate CMOS-compatible\nfabrication, array-level validation, and selector-free suppression of sneak paths—\nadvantages that this work does not yet match. The novelty of a “neutral oxygen”\nreservoir is intriguing but must be proven chemically to stand as a clear advance over\nthese prior works.\nAnswer: We appreciate the reviewer’s valuable suggestions. Regarding the spatial\ndistribution of oxygen, the proposed EOR-based memristor aligns with the design\nprinciples of previously reported devices constructed by engineering oxygen\nconcentration gradients across heterostructures, such as HfO /HfO /HfO or\nx 2 x\nHfO /ZrO /HfO stacks. To further substantiate this, we have included depth-resolved\n2 2-x 2\nXPS and ELNES analyses to elucidate the chemical nature of oxygen within the EOR\nlayer. The results demonstrate that the EOR layer functions as an effective oxygen\nreservoir.\n8. In conclusion, this manuscript presents a technically strong and potentially impactful\nconcept with outstanding device performance. However, several mechanistic and\ninterpretive claims are currently speculative or insufficiently evidenced. The work\nwould benefit greatly from deeper chemical analysis of the EOR layer, quantitative\nconduction modeling, comprehensive reliability testing, and hardware-level\nneuromorphic validation. If these issues are addressed, the study could reach the broad\nand rigorous standard of Nature Communications. At its current stage, I recommend\nmajor revision. The authors are encouraged to provide quantitative and chemical\nevidence to solidify their mechanistic interpretation and to moderate claims that are\npresently speculative.\nAnswer: We appreciate the reviewer's valuable suggestions. To address the concerns\nregarding the physical nature of the EOR layer, we have provided depth-resolved XPS\n18\nanalysis to quantify the elemental distribution and chemical states within the EOR layer.\nRegarding the switching mechanism, the spatial oxygen distribution in our EOR-based\nmemristor aligns with previously reported multi-layer devices. We attribute the superior\ndevice performance—specifically the low HRS current and ultra-low power\nconsumption—to the synergistic effect between the ultrathin HfO switching layer and\nx\nthe electroneutral EOR layer. Following the reviewer's suggestion, a comprehensive\nsneak-path analysis has been conducted to demonstrate the reliable operation of the\nEOR-based crossbar array. Furthermore, we have clarified all ambiguous statements\nthroughout the revised manuscript. We sincerely thank you for your constructive and\ninvaluable feedback, which has helped us to substantially refine the quality of our work.\nReferences\n1. X. F. Wang, Quan Li, R. F. Egerton, P. F. Lee, J. Y. Dai, Z. F. Hou, X. G. Gong.\nEffect of Al addition on the microstructure and electronic structure of HfO film. J.\n2. Shriram Ramanathan, David A. Muller, Glen D. Wilk, Chang Man Park, Paul C.\nMcIntyre. Effect of oxygen stoichiometry on the electrical properties of zirconia\ngate dielectrics. Appl. Phys. Lett. 20 3311–3313 (2001).\n3. Busch, B.W., Pluchery, O., Chabal, Y.J. et al. Materials Characterization of\nAlternative Gate Dielectrics. MRS Bull. 27 206–211 (2002).\n4. Stemmer, S., Chen, Z.Q., Zhu, W.J. and Ma, T.P. Electron energy-loss spectroscopy\nstudy of thin film hafnium aluminates for novel gate dielectrics. J. Microsc. 210 74-\n79 (2003).\nThis manuscript investigates HfS -based memristive devices. However, the work does\n2\nnot demonstrate substantial advances in either device performance or mechanistic\nunderstanding. Although the paper claims a sub-5-nm switching region, this refers\nmerely to the film thickness rather than the actual device footprint. I initially expected\nlateral device scaling, but the reported vertical dimension does not provide meaningful\nnovelty. The manuscript further claims “high-speed” switching, including the 8 ns / 15\nns switching times emphasized in the title and abstract. In reality, the devices are driven\nby 100 ns pulses, with the current merely reaching a steady level at 8 ns / 15 ns. This\ndoes not constitute stable switching. In the community, a switching speed is typically\naccepted only if the device can reliably toggle under pulses of that duration, and the\nsubsequent voltage/current holding sequence confirms stable filament formation or\nrupture. The same issue applies to the purported “ultralow energy” operation, where\nonly the portion of the current transient up to stabilization is considered—this is not an\nappropriate metric. The MNIST recognition demonstration based on simulations also\ndoes not introduce meaningful novelty.\nAnswer: We sincerely thank the reviewer for their thorough examination of our\nmanuscript and for their valuable comments. Detailed point-by-point responses are\nprovided below. We believe that these revisions have significantly improved the clarity\nand quality of the manuscript. Additionally, we have revised the Introduction section to\nbetter highlight the novelty of this work. We have also addressed specific points\nregarding the advancements of our work, particularly in terms of structural design,\ndevice performance, and mechanistic insights.\nRegarding the advantages of thickness scaling, the reviewer correctly notes that\nthe \"sub-5 nm switching region\" refers to the vertical thickness of the HfO functional\nx\nlayer rather than the lateral device footprint. While lateral scaling is a standard approach\nin CMOS technology to increase integration density, vertical scaling of the functional\nlayer offers a viable alternative route to minimize the operating voltage for low-power\napplications. In contrast, aggressive lateral scaling often suffers from edge effects,\nincreased leakage paths, and non-uniform electric fields, all of which can degrade\nswitching reliability. However, thickness scaling presents significant challenges. As the\nthickness of the switching layer decreases, issues related to the stochastic growth and\nrupture of conductive filaments (CFs) and interface stability become increasingly\nprominent. A primary concern is that thickness variations and surface roughness\nsignificantly impact the electric field distribution across the device—particularly when\nthe thickness falls below 10 nm—thereby increasing the variability of switching\nparameters. In addition, the difficulty in precisely controlling the dissolution of CFs\noften leads to the presence of remnant filaments, which serve as leakage paths and result\nin a high high-resistance state (HRS) current and increased static power consumption.\nMoreover, ultra-thin switching layers are highly susceptible to interdiffusion with the\nelectrodes, a phenomenon that severely degrades device stability, reliability, and\ncycling endurance.\nflat interfaces of 2D materials, a uniform, ultrathin HfO switching layer is formed\nx\nbetween the HfS and MoS layers. Consequently, the migration of V is strictly\n2 2 O\nconfined within a nanoscale region (sub-10 nm). This architecture ensures a\nhomogeneous electric field distribution and facilitates the vertical scaling of the\nswitching layer. Furthermore, the EOR layer and the underlying HfS flake act as\n2\neffective diffusion barriers that inhibit the stochastic diffusion of V , resulting in robust\nO\nendurance and retention performance for the EOR-based memristors.\nNovelty in the operational mechanism: The low high-resistance state (HRS)\ncurrent originates from the spontaneously formed EOR layer, in distinct contrast to the\nintricate engineering of oxygen concentration gradients using multi-layer stacks seen\nin previous reports. The electroneutral EOR layer, adjacent to the ultrathin HfO\nx\nswitching layer, facilitates a dynamic equilibrium of V . This mechanism promotes the\nO\nrupture of conductive filaments (CFs) and serves as an effective leakage barrier in the\nHRS. Consequently, the EOR-based memristors achieve ultrafast operation and an\nextremely low HRS current. Furthermore, the high V concentration and the presence\nO\nof metallic hafnium within the ultrathin HfO switching layer enable forming-free\nx\noperation.\nFigure R1 illustrates the thickness evolution of HfS during its conversion to HfO\n2 x\nvia ozone oxidation. With increasing ozone treatment time, the HfS flake gradually\n2\ntransforms from a partially oxidized state to a completely converted HfO layer. Based\nx\non the initial thickness of HfS (11.17 nm, Fig. R1b) and the final thickness of the fully\n2\noxidized sample (15.13 nm, Fig. R1i) measured by atomic force microscopy, the\nexpansion ratio was determined to be 1.35. To quantify the thickness of the converted\nHfO at various stages, we defined two parameters: the total AFM-measured thickness\nx\n(t) and the thickness of the residual unconverted HfS (x). Their relationship is\n2\nexpressed by the following equation: t=x+1.35×(11.17-x). By determining the value of\nx, the thickness of the formed HfO layer can be precisely derived. This indicates that\nx\nincreasing the ozone treatment time can increase the thickness of the HfO layer.\nx\nx x 2 x\nthe HfO thickness as a function of ozone treatment time. (b-i) Evolution of height profiles with\nx\nvarying ozone treatment time. Each inset shows an AFM image. Scale bar: 2 μm.\nTo elucidate the influence of switching layer and EOR layer thickness on\nmemristor performance, a systematic comparison using devices with identical lateral\ngeometries and electrode materials is presented in Fig. R2. Since the oxidation of HfS\n2\ninto HfO is not a self-limiting process, prolonging the oxidation duration (ozone\nx\ntreatment time) leads to an increased HfO thickness. As the oxidation duration exceeds\nx\n40 min, the high-resistance state (HRS) current exhibits a continuous rise. This is\nprimarily attributed to the increased presence of remnant conductive filaments within\nthe thicker switching layer, which serve as leakage paths and lead to a higher HRS\ncurrent. Moreover, the magnitudes of V and V increase, as a thicker switching\nSET RESET\nlayer requires a higher electric field to induce resistive switching. Therefore, reducing\nthe thickness of the switching layer is effective in lowering the operating voltage,\nthereby promoting low-power operation. Simultaneously, the introduction of the EOR\nlayer enables the attainment of extremely low HRS currents.\nx\ntreatment duration on the electrical characteristics of the EOR-based memristor. (b) Dependence of\nLRS and HRS currents on oxidation duration. (c) Dependence of switching voltages (V and\nSET\nV ) on oxidation duration.\nRESET\nDynamic switching operations were performed using 100 ns voltage pulses, with\nthe current stabilizing within 8 ns and 15 ns during the Set and Reset processes,\nrespectively. These ultra-short intervals represent the intrinsic Set/Reset transition times\nof the memristor [1, 2]. Calculating power consumption based solely on these\ntransitions reflects the instantaneous transition power, rather than the total energy\nconsumed during a complete pulse cycle (which includes the non-switching duration).\nHowever, for practical applications, such a metric lacks practical engineering relevance.\nMoreover, despite variations in device structure, materials, and dimensions across\ndifferent technologies, low leakage current remains a critical and universally\ncomparable performance indicator. This metric is particularly vital when evaluating the\npower budget of circuit-level applications. Therefore, to ensure rigorous reporting, we\nhave removed the broad claim of \"low power consumption\" and instead prioritized the\nspecific discussion of \"low HRS current\" in the revised manuscript.\nThe MNIST dataset serves as a cornerstone benchmark in computer vision for\nevaluating image classification algorithms. Simulations utilizing this dataset are\ncommonly employed to assess the potential of emerging hardware for artificial\nintelligence applications. The primary purpose of our simulation was to highlight the\nperformance advantages of the EOR-based memristor. Although a recognition accuracy\nof 97% was achieved, we agree with the reviewer that the simulation did not account\nfor device non-idealities, such as device-to-device variation, cycle-to-cycle variability,\nor retention degradation. Consequently, we have tempered our claims regarding\nneuromorphic computing, presenting them as \"potential applications\" rather than fully\nrealized demonstrations. The manuscript has been revised accordingly.\nIn summary, the vertical sub-5 nm switching layer represents a significant\nadvancement in device miniaturization. Furthermore, the EOR layer offers new\nmechanistic insights into forming-free, low-leakage resistive switching behaviors.\n1. Device area: The manuscript does not specify the device area, which is critical for\nevaluating performance. Only current density is reported, without the absolute\ncurrent values.\nAnswer: We appreciate the valuable suggestions. The active area of each memristor is\ndefined by the geometric overlap between the top and bottom electrodes, measuring 4.2\nµm × 4.2 µm (Fig. R3a). Consequently, the current density was calculated by\nnormalizing the measured current against this effective area (17.64 µm2). We fully agree\n23\nwith the reviewer that providing both absolute current values and specific device\ndimensions is essential for a rigorous performance assessment. Therefore, we have\nexplicitly stated these parameters in the revised manuscript. Furthermore, to ensure\ncompleteness, I-V characteristics plotting both the absolute current and the normalized\ncurrent density have been included in the Supplementary Information.\nFig. R3 | Electrical performance of the EOR-based memristor array. (a) Optical image of the\nfabricated 5 × 5 crossbar memristor array. (b) Typical I-V curves showing the absolute current. (c)\nCorresponding I-V curves plotted as normalized current density.\n2. Endurance testing: Endurance is typically evaluated using ~200 µs pulses. What\nwould the endurance look like under 8 ns/15 ns pulses? The reported endurance of\n~1,000 cycles is insufficient for practical applications; can it be improved?\nAnswer: We appreciate the reviewer's insightful inquiry regarding the device's\nresponse to nanosecond-scale programming pulses. To address this, we investigated the\nconductance modulation induced by electrical pulses with durations ranging from 10\nns to 40 ns, as shown in Fig. R4. Upon the application of these nanosecond-scale pulses,\nthe memristor exhibited clear and reproducible conductance states, confirming that the\nswitching mechanism can be effectively activated on a much shorter timescale. To\ninvestigate the kinetics of forming-free switching, pulse-width-dependent\nmeasurements were performed by varying the pulse duration from 10 ns to 100 ns (Fig.\nR4e). The results demonstrate that the current increases with increasing pulse width.\nBuilding on these insights, we further evaluated the endurance performance using 100\nns programming pulses. The device demonstrated stable and repeatable switching for\nat least 105 cycles, with negligible degradation in current levels (Fig. R4f). These results\nconfirm that while microsecond pulses induce a larger conductance modulation, the\ndevice remains fully functional under nanosecond-scale pulses, and reliable endurance\nis maintained even with 100 ns pulses. The corresponding experimental results and\ndiscussion have been incorporated into the revised manuscript to elucidate the pulse-\nwidth-dependent programming and endurance behavior.\nresponse to 1.5 V pulses with durations of (a) 10 ns, (b) 20 ns, (c) 30 ns, and (d) 40 ns. (e)\nConductance modulation induced by pulses with durations ranging from 10 ns to 100 ns. (f)\nCorresponding endurance characteristics measured using SET/RESET pulses of ±1.5 V (100 ns\nduration).\n3. Retention testing: Retention should not be evaluated only at room temperature.\nMeasurements at 85 °C—or preferably 125 °C—are standard practice.\nAnswer: We appreciate the reviewer's valuable suggestion regarding retention\ncharacterization at elevated temperatures. To address this point, we conducted retention\nmeasurements at 85 °C and 125 °C, which are critical benchmarks for evaluating the\nlong-term stability of non-volatile memory devices. The results demonstrate that the\ndevice exhibits excellent retention performance under thermal stress (Fig. R5). Both the\nHRS and LRS remained stable over a duration of 105 s, indicating that the switching\nstates are thermally robust and exhibit negligible degradation at these elevated\ntemperatures. We have incorporated these data and the corresponding discussion into\nthe revised manuscript.\nFig. R5 | Retention characteristics of the EOR-based memristor at elevated temperatures. (a)\nRetention measured at 85 °C. (b) Retention measured at 125 °C.\n25\n4. Memory window: The experimental results indicate a memory window of\napproximately 3–5 orders of magnitude, with noticeable variation across tests. This is\nfar from the 10-order-of-magnitude window claimed in the manuscript.\nAnswer: We appreciate the valuable suggestion. To ensure statistical rigor, we extracted\nthe HRS and LRS currents from the I-V curves of 100 devices (measured at V = 0.1\nread\nV) and calculated their ON/OFF ratios. As shown in Fig. R6, while a maximum\nswitching ratio of 1010 was observed, we opted for a more conservative evaluation.\nBased on the statistical analysis, the average HRS current density is 1.52 fA/µm2, while\nthe average LRS current density is 0.27 μA/µm2. Therefore, to more accurately reflect\nthe reliability and reproducibility of the device performance, we report an average\nON/OFF ratio of 1.47×109. We have updated the manuscript to reflect these statistical\nperformance metrics.\nFig. R6 | Statistical switching performance of the EOR-based memristors. (a) Cumulative\nprobability distributions of the HRS and LRS currents. (b) Cumulative probability distribution of\nthe on/off ratio extracted at V = 0.1 V.\nREAD\n5. Inconsistencies in resistance values: The reported high-/low-resistance states vary\nsignificantly across Fig.s. In Fig. 2b they are 10-12 / 10-16 A, in Fig. 3h they become 10-\n13 / 10-16 A, while in Fig. S3 they are around 10-16 / 10-7 A. These inconsistencies need\nto be clarified and justified.\nAnswer: We appreciate the reviewer's careful examination of the resistance values\nreported in the manuscript. The observed variations originated from differences in\nsample sizes across the datasets. Specifically, the original Fig. 2b, which depicts the\nevolution of current as a function of oxidation duration, presented average values\nderived from an initial subset of 25 devices within a single 5 × 5 array. This relatively\nsmall sample size accounted for the previously reported lower average HRS current.\nIn contrast, Fig. R6a presents the statistical distribution of HRS and LRS currents\ncollected from 100 devices. To ensure accuracy and statistical representativeness, we\nhave recalculated the average values and error bars for the 40-minute ozone treatment\ngroup using this larger dataset. Fig. 2b has been updated in the revised manuscript to\nreflect these corrected statistics.\nthe data were acquired at a read voltage of V =0.2 V, which differs from the condition\nRead\nused in Fig. 2b. We selected V = 0.2 V for this measurement because the\nRead\ntemperature-dependent variation in HRS current is less distinguishable at 0.1 V due to\nthe low current level. A higher read voltage allows for a clearer visualization of the\nconduction mechanisms. To prevent ambiguity, we have explicitly specified the read\nvoltage in both Fig. 3h and its corresponding caption in the revised manuscript.\nFurthermore, we acknowledge that the initial temperature-dependent data were\nobtained using an Agilent 2912 semiconductor analyzer, which has a higher noise floor\ncompared to the Keithley 4200-SCS used for testing the afore mentioned I-V curves.\nThe background noise current measured using the two instruments is shown in the Fig.\nR7. This was the primary reason for the discrepancy in off-state current values.\nHowever, the evolution trend of the electrical performance of the device with\ntemperature is the same. To ensure total data consistency across the manuscript, we\nhave re-measured the temperature-dependent performance using the Keithley 4200-\nSCS and updated Fig. 3h accordingly.\nFigure R6a presents the statistical current distributions collected from 100 devices.\nThe calculated average current densities for the HRS and LRS are 1.52 fA/µm2 and\n0.27 µA/µm, respectively, which align closely with the values noted by the reviewer.\nFurthermore, these results are consistent with the other experimental data presented in\nthe manuscript.\nWe sincerely thank you for your meticulous examination of our manuscript. We\nhave revised the paper significantly to incorporate your constructive comments.\nFig. R7 | Background noise current of the (a) 4200A-SCS and (b) B2912A semiconductor analyzers,\nrespectively.\nReferences\n1. Teja Nibhanupudi, S.S., Roy, A., Veksler, D. et al. Ultra-fast switching memristors\nbased on two-dimensional materials. Nat. Commun. 15, 2334 (2024).\n2. Wang, G., Li, S., Wang, C. et al. Structural Transformation of 2D InSe Toward\nUltrafast and Energy-Efficient Non-Volatile Memristive Switching. Adv. Funct.\nMater. e16141 (2025).\n27"}, {"round": 2, "reviewer_comments": {"reviewer_1": "After careful assessment, the authors have addressed all of the concerns I raised. I therefore recommend that the manuscript be accepted for publication in Nature Communications.", "reviewer_3": "The authors have addressed my concerns."}, "author_response": ""}]

Decision letter