Article

https://doi.org/10.1038/s41467-024-44970-4

Pulse irradiation synthesis of metal
chalcogenides on ﬂexible substrates forenhanced photothermoelectric performance
Received: 28 May 2023

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Accepted: 11 January 2024

Yuxuan Zhang 1, You Meng 2 , Liqiang Wang3, Changyong Lan4, Quan Quan1,
Wei Wang1, Zhengxun Lai1, Weijun Wang1, Yezhan Li1, Di Yin1, Dengji Li1,
Pengshan Xie1, Dong Chen1, Zhe Yang5, SenPo Yip6, Yang Lu 7,
Chun-Yuen Wong 5 & Johnny C. Ho 1,2,6

High synthesis temperatures and speciﬁc growth substrates are typicallyrequired to obtain crystalline or oriented inorganic functional thin ﬁlms,
posing a signiﬁcant challenge for their utilization in large-scale, low-cost(opto-)electronic applications on conventional ﬂexible substrates. Here, we
explore a pulse irradiation synthesis (PIS) to prepare thermoelectric metal
chalcogenide (e.g., Bi2Se3, SnSe2, and Bi2Te3) ﬁlms on multiple polymeric
substrates. The self-propagating combustion process enables PIS to achieve asynthesis temperature as low as 150 °C, with an ultrafast reaction completed
within one second. Beyond the photothermoelectric (PTE) property, thethermal coupling between polymeric substrates and bismuth selenide ﬁlms is
also examined to enhance the PTE performance, resulting in a responsivity of
71.9 V/W and a response time of less than 50 ms at 1550 nm, surpassing most of
its counterparts. This PIS platform offers a promising route for realizing ﬂexible PTE or thermoelectric devices in an energy-, time-, and cost-efﬁcientmanner.

Scalable thin ﬁlm deposition techniques, including chemical (physical)
vapor deposition and solution-processed coating, have been developed to produce amorphous or nanocrystalline ﬁlms1–5. However,additional high-temperature processing steps (typically above 400 °C)
are necessary when crystalline ﬁlms are required6,7. This requirement
can pose signiﬁcant challenges in achieving desirable features ofcrystalline ﬁlms while working with thermally unstable substrates andother device components8–10. In particular, polymeric substrates,
which are extensively used in ﬂexible electronics, are heat-intolerant
and atomically disordered, rendering them incompatible withhigh-temperature treatment and high-quality material growth.

Consequently, the performance of electronic devices fabricated onsuch substrates is frequently compromised by the presence of inferior
crystallinity and lattice defects4,11–13. Thus, there is a pressing demand
for thin-ﬁlm processing methods that enable direct low-temperaturedeposition while still providing high-quality semiconductor
characteristics.
Although ﬂexible substrates are often deemed simply as platforms to support device components, their properties can signiﬁcantly
impact the overall device performance if their thermal impacts takeeffect. Unlike solid-state inorganic materials, polymeric matrixes
usually consist of disordered long chains with low crystallinity, giving

1

Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR 999077, P.R. China. 2State Key Laboratory of Terahertz andMillimeter Waves, City University of Hong Kong, Hong Kong SAR 999077, P.R. China. 3Department of Mechanical Engineering, City University of Hong Kong,
Hong Kong SAR 999077, P.R. China. 4State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology ofChina, Chengdu 610054, P.R. China. 5Department of Chemistry, City University of Hong Kong, Hong Kong SAR 999077, P.R. China. 6Institute for Materials
Chemistry and Engineering, Kyushu University, Fukuoka 816 8580, Japan. 7Department of Mechanical Engineering, The University of Hong Kong, Hong Kong
SAR 999077, P.R. China.
e-mail: youmeng2@cityu.edu.hk; acywong@cityu.edu.hk; johnnyho@cityu.edu.hk

Nature Communications | (2024)15:728

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Articlehttps://doi.org/10.1038/s41467-024-44970-4

rise to a thermal insulating characteristic14,15. In this regard, the temperature distribution on polymeric substrates is susceptible to small
amounts of energy input. Therefore, rational substrate thermal management, such as selecting a ﬂexible substrate with suitable thermal
properties, can be a promising strategy for designing thermalmediated devices. For example, detectors based on the PTE effect,
which arises from the photo-induced thermal effect and the Seebeck
effect, are well known for their broadband responses16. Previous studies have reported that the responsivity of PTE detectors can be
improved by utilizing electrical gating, surface plasmonics, antenna
coupling, and phonon absorption, which result in a higher temperature gradient or enhanced Seebeck coefﬁcient17–21. In the meantime, aprior work has pointed out that the underlying solid substrates canalso inﬂuence the PTE response via thermal effects22. Thus, unveilingthe thermal coupling effect between the unexplored polymeric substrates and functional ﬁlms is essential to improve device
performance.
Herein, we effectively lower the processing temperature of metal
chalcogenides (e.g., Bi2Se3, Bi2Te3, and SnSe2) down to 150 °C via apulse irradiation process, unachievable in other conventional techniques. By carefully tuning the irradiation and temperature proﬁles, andthe structure of the reactant layer, we achieve an ultra-short reaction
time of less than one second, demonstrating the energy efﬁciency of
PIS. It is proposed that the self-propagating combustion process is
responsible for ultrafast low-temperature synthesis. Compared to nonPTE photodetectors, our PTE-based ﬁlm photodetectors produced by
PIS exhibit good compatibility with ﬂexible substrates and bandgapindependent wide spectral response. Importantly, we discover asynergistic thermal effect between polymeric substrates and PTE ﬁlms,
which offers an alternative route for improving the PTE with wide-band
detection capability.

Results
Bi2Se3 thin-ﬁlm characterizationab
Alternating layers

Bi
Se
Bi

Temperature (°C)

The Bi2Se3 ﬁlm is prepared by stacked elemental layers with a subsequent pulse irradiation synthesis (Fig. 1, see Methods for detailed
information). Speciﬁcally, selenium (Se) and bismuth (Bi) layers aresequentially deposited in an ultra-high vacuum physical vapor

deposition chamber. The Bi layer is always deposited atop the Se layer
to prevent Se loss during PIS23. In a typical PIS process, the highintensity radiation generated from the tungsten halogen lamp heats
the sample, and the temperature rises to a preset point within a few
seconds. The precisely controlled temperature system enables fast
ramping rates (~100 K s−1) with a heating duration of less than one
second (upper panel of Fig. 1b). Consequently, the pulse irradiation
triggers the interdiffusion between layers, and Bi2Se3 nucleates andgrows into a layered structure. In the meantime, a mass ﬂow of inert Ar
is used to prevent the ﬁlm from possible oxidation.
Grazing-incident wide-angle X-ray scattering (GIWAXS) patternsin Fig. 2a–d reveal that the ﬁlm has a uniaxial texture with a crystallographic c-axis aligned perpendicularly to the substrate surface.
Even if the PIS temperature is as low as 150 °C, Bi2Se3 still emerges
with diffractions of (003) and (006) at qz = 6.6 nm−1 and 13.2 nm−1
(Fig. 2a)24. The preferential orientation becomes prominent as the PIS
temperature increases to 300 °C (Fig. 2b–d and Supplementary Fig. 1),
which suggests that the prime crystallite extends along the substratesurface and the grain sizes increase with temperature (Supplementary
Fig. 2). The structural evolution is detailed in Raman spectra (Supplementary Fig. 3 and Supplementary Note 1) and X-ray photoelectron
spectroscopy (Supplementary Fig. 4)25. High-resolution transmission
electron microscopic (HR-TEM) images further reveal the transformation process. Figure 2e illustrates clear interfaces between Bi and
Se elemental layers before PIS. Corresponding selected-area electron
diffraction patterns (SAED) show the halo feature with diffraction
rings, which are associated with amorphous Se phases and tinypolycrystalline Bi layers (Fig. 2f). After PIS, the well-crystallized Bi2Se3
ﬁlm is obtained, and the layered structure is as well distinguishable in
the SAED image (Fig. 2g, h). It is noted that grain boundaries can beobserved from a larger view, suggesting that the ﬁlm is not a perfect
single crystal (Supplementary Fig. 5).
Further insight into the layered structure crystallization during
PIS can be obtained through the density functional theory-based
molecular dynamic (MD) simulation. In a simulated annealing processof the Bi/Se layered structure, typical snapshots during MD trajectories
show the interdiffusion that initiates from the interface and a rapid
fusion that almost ﬁnishes after 18 ps (Fig. 2i and Supplementary

Duration < 1 s

150c

Cooling100

Heating ~100 K s-1

50
0

5

10 15
Time (s)20
Bi2Se3

1 QL

Flexible substrates

Bi
Pulse irradiation (150 C)

Fig. 1 | Schematic of pulse irradiation synthesis (PIS) for highly conformable
photothermoelectric (PTE) ﬁlms. a In PIS, alternating elemental layers are initiallydeposited on ﬂexible substrates. b The ﬁlm is then subjected to pulsed irradiation,
which induces rapid heating of the ﬁlm at a rate of approximately 100 K s−1. The

Nature Communications | (2024)15:728

Se

temperature proﬁle in a typical PIS is shown in the upper panel. c Owing to the rapid
heating, crystallization is initiated at a signiﬁcantly lower temperature (e.g., 150 °C),
thereby preserving the ﬂexibility of the substrates. Quintuple layers (QL) of the
Bi2Se3 crystal structure are shown in the lower panel.

2Article5d 15
qz (nm-1)

(006)

c 15
qz (nm-1)

qz (nm-1)

a 15
10

b 15
qz (nm-1)

https://doi.org/10.1038/s41467-024-44970-4

10
5

10
5

10
5

(003)
150 C

0
-5

0
5
qxy (nm-1)

e

10

Se

200 C

0
-5

0
5
qxy (nm-1)

250 C

0

10

-5

0
5
qxy (nm-1)

g

f

10

-5

0
5
qxy (nm-1)

10

h

(104)
Bi

30 0 C

0

9.3 Å

(012)

(006)

(003)

Se
Bi
Se

20 nm

20 nm

2 1/nm2 1/nmjiPrimary
clusterformation

Bi
Se
0 ps

6 ps

18 ps 20 ps Time 0 ps

1 ps

13 ps 15 ps

Fig. 2 | PIS synthesis of large-area PTE ﬁlm and characterization. a–d Grazingincident wide-angle X-ray scattering (GIWAXS) patterns of Bi2Se3 ﬁlms after PIS at
150 (a), 200 (b), 250 (c), and 300 °C (d) for 1 s, respectively. White arcs in (a)represent diffractions of Bi2Se3 with a preferential out-of-plane orientation.
e, f High-resolution transmission electron microscopic (HRTEM) image andselected-area electron diffraction (SAED) pattern of the ﬁlm after evaporation. The
white dashed rings in (f) correspond to diffractions of polycrystalline Bi layers.

g, h HRTEM image, and SAED pattern of the ﬁlm after 300 °C PIS, showing the (003)interplanar spacing (g) and (006) diffraction spot of Bi2Se3 (h). i, j Typical snapshots taken in the ab-initio molecular dynamic (AIMD) simulation during theinterdiffusion of Se and Bi layers (i) and attachments of Se and Bi atoms to the edgeof the primary Bi2Se3 cluster (j), almost completing two columns of atoms on bothsides (highlighted with colored boxes).

Movie 1). Once the primary cluster is formed, Bi or Se atoms approachthe edge of the Bi2Se3 cluster and promote its lateral crystal growth(Supplementary Movie 2). These atoms almost complete two new rowsof Bi2Se3 after 1 ps, while the layered structure is further stabilized after
13 ps (Fig. 2j). This supports the energy-favorable PIS-mediated rapid
nucleation and lateral growth of layered Bi2Se3.

ignition temperature is conﬁrmed by the differential scanning calorimetry (DSC) curve, where the exothermic peak indicates that theignition temperature is 150 °C (Supplementary Fig. 6a), consistent withprevious observations. Interestingly, the ignition temperature in thiswork is lower than that of conventional combustion synthesis of bulk
metal chalcogenides (generally 220–350 °C)27–29, which can be attributed to the kinetic process (i.e., thermal diffusion)30. We estimate thecharacteristic diffusion distance of Se using the one-dimensional semiinﬁnite model, yielding a result of ~1.1 μm (Supplementary Note 2),
which covers the entire thickness of the ultrathin elemental ﬁlm(10 nm). Combined with a small formation barrier of 0.11 eV (Supplementary Fig. 6d), the self-propagating combustion can be triggered ata relatively low temperature and completed within a few seconds
via PIS.
With the low-temperature PIS, the rapid heating/cooling process
helps bypass the thermal-induced degradation of ﬂexible substrates.
After the conventional furnace annealing (CFA) at 150 °C, the polyethylene terephthalate (PET) deforms and losses its integrity, while no
deformation is observed after PIS at the same temperature (Supplementary Fig. 7). Furthermore, the PIS yields a dense, ﬂat, and continuous ﬁlm as conﬁrmed by scanning electron microscopy (SEM),
which is in contrast to the CFA treated ﬁlm with apparent porosity andMechanism of pulse irradiation synthesis
A self-propagating combustion process is proposed for this ultrafast
and low-temperature synthesis. In this work, the exothermic reactionof 2Bi + 3Se → Bi2Se3 exhibits a negative formation enthalpy (~ −140 kJ
mol−1), and the calculated adiabatic temperature is ~722 °C, which ismuch higher than the melting temperatures of both Bi (271.4 °C) andSe (220.8 °C), satisfying the prerequisite for sustaining the propagation of the combustion wave26. Upon ignition of the stacked Bi/Se
layers by an energy pulse, the heat released from the exothermic
reaction outpaces the heat dissipation rate from the reactants, causing
the local temperature to rise near the adiabatic temperature. Thus, acombustion wave passes through the ﬁlm as the self-generated heat is
sufﬁcient to maintain the reaction until the complete consumption of
the reactants. This phenomenon eliminates the need for extra energy,
signiﬁcantly reducing the processing temperature. Furthermore, theNature Communications | (2024)15:728

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Articlehttps://doi.org/10.1038/s41467-024-44970-4

cracks (Supplementary Fig. 8a–e). At the same time, the PIS also avoids
the Se sublimation loss due to its high saturated vapor pressure, whichis conﬁrmed by the compositional depth proﬁling of the ﬁlm (Supplementary Fig. 8f, g)31. For the same reason, the ﬁlm thickness remains
nearly unchanged after PIS (Supplementary Fig. 9). Surface morphologies are then examined by atomic force microscopy (AFM), revealingthat PIS-treated samples exhibit a smooth surface, with the standard
deviation of roughness (Ra) slightly increasing from 2.60 nm (150 °C)
to 3.36 nm (400 °C) as the temperature rises (Supplementary Figs. 10
and 11).

substrate is the main factor that inﬂuences the temperature (T) distribution (Supplementary Note 3), which can be described by:∂T= α∇2 T∂t

Figure 3a, b illustrates the simulated temperature distribution
with a device conﬁguration shown in Fig. 3c. Due to the signiﬁcantly
different thermal diffusivity (0.21 and 0.87 mm2 s−1 for PI and SiO2,
respectively), a temperature distribution of −0.317 K μm−1 is found on
PI, larger than −0.036 K μm−1 of SiO2 (Fig. 3d, e). Accordingly, thedifferent temperature gradients lead to a larger photoresponse of the
PI-based device (1.20 mV) than the SiO2-based one (0.08 mV) (Fig. 3f).
Beneﬁting from facile deposition, the PIS is readily compatible withmultiple polymeric substrates, such as polydimethylsiloxane (PDMS),
PET, and poly(ethylene 2,6-naphthalate) (PEN). The compiled

Thermal coupling between substrates and PTE ﬁlms
After that, the thermal coupling effect between polymeric substrates
and metal chalcogenide ﬁlms is explored. We ﬁnd that the photoresponsivity is closely related to the magnitude of the temperature
gradient within the substrate, and the thermal diffusivity (α) of the

a

PI
SiO2

d
308

Temperature (K)S

D

100 μm

2 mm

ð1Þ

T (K)

Channel

S

D

304
300
296
292
0

295

293

20

40

60

Distance along channel (μm)

eTemperature gradient (K/μm)

S

D

100 μm

T (K)

PI
SiO2

1.4

ChannelElectrode

Vph (mV)

1.2

Electrode

1.0
0.8
0.6
0.4
0.2

20 μm

1
0

300

293
f

c

2

0

20

40

60

Distance along channel (μm)

g350Responsivity (V/W)

2 mm

3

0.25
300

0.20

250

0.15

200
150

0.10

100

0.05

Diffusivity (mm2 s-1)

b

50

0.0
0

5

10

15

20

25

PDMS

PET

PEN

PI

Time (s)Fig. 3 | Thermal management of polymeric substrates. a, b Thermal simulations
on SiO2 (a) and PI (b) surfaces based on the device architecture. S and D denotesource and drain electrodes, respectively. c Optical image of the Bi2Se3 ﬁlm photodetector. d, e Corresponding temperature (d) and gradient (e) proﬁles across thedevice channel. Two vertical dashed lines indicate the interfaces between electrodes and channels. f Temporal photovoltage (Vph) response curves comparing the

Nature Communications | (2024)15:728

PTE ﬁlm on Si/SiO2 and PI substrates. g Responsivity on four typical ﬂexible substrates, i.e., polydimethylsiloxane (PDMS), polyethylene terephthalate (PET),
poly(ethylene 2,6-naphthalate) (PEN), and polyimide (PI), with the summary of thecorresponding diffusivity (Supplementary Table 1), indicating an inverse correlation between them. Error bars indicate standard deviations of responsivities
obtained from 30 individual devices.

4Articlehttps://doi.org/10.1038/s41467-024-44970-4

PTE response of metal chalcogenide ﬁlms

performance is also consistent with the simulation result (Supplementary Fig. 12). Therefore, using substrates with lower thermal diffusivity can effectively improve the photovoltages and responsivities(Fig. 3g and Supplementary Table 1). It is noted that all ﬂexible substrates show poor absorption with minor differences in absorption
spectra, whereas the Bi2Se3 ﬁlm demonstrates higher absorption(Supplementary Fig. 13), which further underscores that the device
performance is primarily governed by the inherent thermal propertiesof the underlying substrate itself, instead of its optical absorption. In
addition, surface meta-materials can be employed as plasmonic
absorbers to improve performance32. It is observed that the photoresponse increases ~60% after integration with gold-based metamaterials (Supplementary Fig. 14). This result may prompt further
investigations into the synergistic effect of thermal coupling and surface plasmonic absorption.

a

For PTE devices, when the laser focuses on one side of the channel,
the temperature gradient is built, which drives the diffusion ofelectrons from the warm side to the cold side (major carriers in
Bi2Se3 are electrons). As a result, we can observe a negative electrical potential (Fig. 4a). Unlike traditional photoconduction
detectors, the PTE photovoltage can be changed or even reversedby altering the temperature gradient along the channel throughmanipulation of laser illumination position. The scanning photovoltage mapping (SPVM) directly exhibits an opposite photovoltage(Vph) of ±180 μV at two ends of the channel (Fig. 4b). Meanwhile, azero Vph crossover position is found located at the center area of thechannel. Such zero-Vph zone can be shifted when an external bias
(Vds) is applied and ﬁnally disappear at a Vds of 200 μV (Fig. 4c andSupplementary Fig. 15), agreeing well with the typical PTE

b
Au

c

200

400

100

i

300

150
100

0
-100
-200

50
0
-50

200
100
0

-100

ii

100

-150

-100

iii

0

40

20

60

-200

X posit

80

10
0

20

Y position (μm)

ion (μm

)

80

60

40

-200
100

0

Vph (mV)

-0.6

iii

0.2
0.1

3

-1.0
S = -111 μV K-1
R2 = 0.999

-1.4
-1.6

iV

V

2
1
0

-1.8
0

2

4

6

8

10

12

14

16

0

1

2

ΔT (K)g
Positive
Negative

Responsivity (V/W)

Vph (mV)

1.0

0.5

0.0

-0.5

4

h
72

102

2.0

70
1.5
68

α=1

66
64

1.0

0.5

Responsivity (V/W)

light onlight off

3

Time (s)Vph (mV)

f
1.5

0

0.0

-0.8

-1.2

Position20

0.3

ΔT (K)ii

i

ΔU (mV)

V

40

0.4

-0.2
-0.4

e-

60

e 0.5

0.0

iV

80

Distance along channel (μm)

d

ΔV ΔV=−S∙ Tl −Tr

0V
50 μV
100 μV
200 μV

200

Au

Vph (μV)e-

Vph (μV)

T[20]

This work

[37]

101

[40]
100

[38]

[39]
[22]

10-1

[41]
[42]

62

-1.0

0.0
0

20

40

60

80

100

5

Time (s)Fig. 4 | Mechanism and performance of PTE photodetectors. a Schematic of thelocal laser-induced temperature distribution T and potential distribution ΔV along
the channel when the laser spot moves from the left electrode to the right one. Red
triangle cursors indicate the position where the focused laser is illuminated in thedevice channel. The white arrow indicates the diffusion direction of hot electrons.
S, Tl, and Tr refer to the Seebeck coefﬁcient and temperatures on the left and rightelectrodes, respectively. b Scanning photovoltage mapping (SPVM) in a 3D plot.
Values projected on the bottom surface show a symmetric distribution of Vph. Graydashed lines indicate the channel-electrode interface and the solid grey line refers
to the contour with zero Vph. c Line proﬁles of the Vph cutting along the channel

Nature Communications | (2024)15:728

10

15Power (μW)

20

10-2
400

800

1200

1600

Wavalength (nm)under increasing Vds with the dashed line indicating the zero Vph. d Voltage acrossthe Bi2Se3 ﬁlm versus the corresponding temperature difference to determine its
room-temperature Seebeck coefﬁcient S. The orange line is the linear ﬁtting of theexperimental data, with a coefﬁcient of determination being 0.999. e Comparisonof the Vph-time curve and the ΔT-time curve. f Temporal response with the laser
spot positioned on either side of the device, showing a positive and a negative
photoresponse. g Responsivity and Vph as a function of incident power under
1550 nm illumination. The dashed line is a reference line whose ideal factor α equals
to 1. h Benchmark of the photodetector of reported PTE detectors.

5Articlehttps://doi.org/10.1038/s41467-024-44970-4

mechanism. According to the Seebeck effect, the photovoltage is
generated by photo-induced temperature difference (ΔT) as:
V ph =  S  ΔT

ð2Þ

where S is the Seebeck coefﬁcient. It characterizes the ability of amaterial to convert a temperature difference (ΔT) to an electric
potential difference (ΔU)33, and it is extracted to be -111 μV K-1 (Fig. 4d),comparable with other Bi2Se3 ﬁlms synthesized with conventional
methods (Supplementary Table 2) and other chalcogenide
counterparts34–36, but with a lower processing temperature and ashorter processing time. It is worth mentioning that the unique photoresponse behavior of PTE distinguishes it from other mechanisms,
such as the photovoltaic effect and photo-Dember effect (Supplementary Note 4). Recording by a thermal infrared camera (Supplementary
Movie 3), a 1550 nm laser irradiation can induce ~3 K temperature
difference on the channel surface, by which a Vph of ~0.4 mV is
produced simultaneously (Fig. 4e), corresponding well with the
Seebeck coefﬁcient test (Fig. 4d). In addition, the PTE performancecorrelates with the fabrication process. The appropriate ﬁlm thickness
and processing temperature lead to an improved crystallinity and an
optimized output Vph (Supplementary Fig. 16 and Supplementary Note 5).
Next, we systematically investigate PTE detectors on the PI substrate (Supplementary Fig. 17). The temporal Vph exhibits bi-directional
positive/negative responses with well-symmetric characteristics whenthe 1550 nm laser illuminates two ends of the channel (Fig. 4f and
Supplementary Fig. 18). The rapid and reproducible response undercycling switches indicate good stability in the near-infrared detection,with a high on/off ratio of ~102 (Supplementary Fig. 19). As we raise thelight power (P), the Vph is proportionally increased (Fig. 4g and Supplementary Fig. 20), while the extracted channel resistances keep
constant (Supplementary Fig. 21). The linear relationship between P
and Vph can be ﬁtted by the power law equation: V ph / P α , where theﬁtted ideal factor α is close to 1. These results rule out the possibility ofphotoconductive and bolometric effects contributing to photoresponse in this work. The corresponding responsivity (R = Vph/P)
remains at the same level with different P and peaks at 71.9 V/W for
1550 nm IR light, surpassing most PTE detectors (Fig. 4h and Supplementary Table 3)20,22,37–42. Given the zero bias in the measurement, theshot noise in the dark current is negligible, whereas the ﬂicker noise
and Johnson noise will dominate the dark current in the PTE device43.
The power-dependent detectivity (D*) extracted from the noise
equivalent power (NEP) reaches a maximum value of 1.85 × 107 Jones at1550 nm (Supplementary Figs. 22 and 23), comparable with somerecently reported PTE photodetectors44,45. The performance of the
Bi2Se3-based PTE detector can be further improved through material
doping, rational electrode design, plasmonic nanoantenna, hybrid
structure, and so on46–50.
As an important ﬁgure-of-merit for photodetectors, the rise anddecay times are extracted to be 49.7 and 49.5 ms, respectively, whichindicates an estimated 3 dB cutoff at ~20 Hz (Supplementary Fig. 24a, b).
The scale of the PTE response time reﬂects the phonon-dominatedtransport in the Bi2Se3 ﬁlm and the thermal equilibrium rate that largely
depends on the thermal conductivity and the heat capacity ofmaterials43. Under external bias, the photoconductive effect starts tocontribute to the response (Supplementary Fig. 24c, d), and carrier
scattering and electronic trapping are responsible for the relatively
longer response time. Besides, in Supplementary Fig. 25, after 5000 s ofrepeated illumination, continuous illumination for 120 s, or 6-monthambient storage, there is hardly any discernable degradation of thephotovoltage, signifying its robust detection stability. Also, the device
maintains a reproducible photoresponse characteristic, with no discernable degradation of responsivity (~5%) after 5000 bending cycles(Supplementary Fig. 26).

Nature Communications | (2024)15:728

Based on the mechanism of PTE, the detection wavelength in our
proposed strategy is not limited by the bandgap of the active ﬁlm. The
PIS-treated ﬁlms can realize wide-spectrum detections, by which theaverage responsivities under 405, 450, 532, and 635 nm illuminations
are 75.8, 55.1, 52.4, and 40.0 V W−1, respectively (SupplementaryFigs. 27 and 28). Finally, to show the broad generality of PIS, SnSe2 andBi2Te3, which are typical thermoelectric materials usually obtained
under high temperatures34,51, are fabricated on PI via 150 °C PIS. Theseﬁlms with PIS exhibit good PTE response under IR illumination (Supplementary Fig. 29).

Discussion
In summary, a low-temperature pulse irradiation method is explored
to produce metal chalcogenide ﬁlms for ﬂexible PTE devices. In atypical PIS, the self-propagating combustion process triggered by anenergy pulse effectively avoids high temperatures, and the resultant
combustion wave rapidly passes through the thin ﬁlm, allowing thematerial to crystallize within one second. Powered by the selfpropagating combustion process, the PIS achieves a low synthesis
temperature of 150 °C and completes the ultrafast reaction within one
second, leaving minuscule energy overhead. Also, the synergetic effectbetween polymeric substrates and bismuth chalcogenides ﬁlms
improves the PTE performance through substrate-related thermal
management. It is found that thermal diffusivities of different substrates correlate with their temperature proﬁle under local illumination, which couples with the thermoelectric ﬁlm and affects the output
photovoltages. The low temperature and rapid process in PIS allow us
to use ﬂexible substrates with suitable thermal properties. In thisregard, we present a pathway that further manipulates the performance of PTE devices. Moreover, the wide spectrum photoresponse,high responsivity (71.9 V/W for 1550 nm), and fast response (<50 ms)
surpass most of the counterparts. The PTE effect, which includes two
processes: the photothermal conversion and the Seebeck effect, provides a powerful platform that transfers optical signals to electricalreadouts without the limitation of the bandgap of active materials.
Thanks to the low processing temperature, PIS can be well generalized
to a broad range of ﬂexible substrates and materials, demonstrating its
potential for the future development of ﬂexible PTE photodetectors
and thermoelectric devices.

Methods
PIS treatment and device fabrication
Elemental layers were thermally evaporated on the substrate in analternative manner. For the Bi2Se3 ﬁlm, the evaporation rate of Bi and Se
was controlled to be around 0.5 Å s−1. The thicknesses of the Bi and Se
layers were 10 and 15 nm, respectively. The pattern of the photodetection ﬁlm was deﬁned by a shadow mask. Then, the PIS was realized
in a rapid thermal process system (AccuThermo AW410). A susceptor
coated with inert silicon carbide was used to encase the substrate with
evaporated ﬁlms. The ramping temperature was ~100 K s−1 with a steadytime of less than 1 s. Thus, the synthesis time was 2 s and 4 s for PIS
temperatures of 150 and 300 °C, respectively. To avoid the overshootand undershoot of the temperature proﬁle and ensure repeatability, allparameters were carefully tuned. During the treatment, high-purity Ar
with a ﬂow rate of 10 standard liters per minute (SLPM) was introduced
into the system to protect the sample. Photodetectors were fabricatedusing metal lithography stencils to deposit the source and drain electrodes. The laser-cut mask with a distinct channel length of 60 μm was
manually aligned on top of the pre-patterned photodetector ﬁlm via an
optical microscope. A 50 nm gold electrode layer was then deposited
through thermal evaporation (rate: 1 Å s−1).

Material characterization
The texture of the ﬁlm was characterized by GIWAXS using a smallangle X-ray scattering system (Xeuss 3.0). The sample-detector
6Articlehttps://doi.org/10.1038/s41467-024-44970-4

distance was set to 120 mm, and the exposure time was 1800 s. The
phase composition was studied from the XRD (D2 Phaser X-ray Diffractometer System). The surface morphology was characterized by
SEM (Quanta 450), and the crystallography structure was determined
by TEM (CM-20, Philips). The ﬁlm topography and the roughness were
evaluated by AFM (MultiMode 8, Bruker). The Raman spectra were
measured by the confocal microscope spectrometer (Alpha 300R,
WITec). For the investigation of the surface chemical state and theatomic ratio of the ﬁlm, XPS was employed using a Thermo Fisher
ESCLAB Xi+ system. The depth proﬁling of the elemental ratio wasrealized by Ar+-etching, with each etching depth controlled toapproximately 10 nm. The thermal analysis during heating was realized
by a differential scanning calorimeter (DSC 8000, PerkinElmer). A
stoichiometric amount of high-purity Bi and Se powder was weighted
and mixed in an agate mortar and then transferred into an aluminum
pan, which was then cold pressed and sealed via a universal crimperpress. In order to approximate the PIS process as closely as possible,
the pellet was heated at a rate of 100 K min−1. The Seebeck coefﬁcient
was obtained from a thermoelectric material test system (CTA-3,
Cryoall). Before measurement, two thermocouples were connected tothe ﬁlm to obtain the voltage-current curves at room temperature. The
test proceeded when the coefﬁcient of determination for the ﬁtted
voltage-current curve exceeded 0.99999, which indicated good contact of the thermocouple with the surface. A heater was employed tocreate a temperature gradient in the ﬁlm. Once the temperature difference stabilized, temperatures at the two thermocouple locations
and the electrical potential between them were measured simultaneously. Then, the Seebeck coefﬁcient was extracted from the ΔU-ΔT
curve (Fig. 4d).

Ab initio calculation was carried out within the DFT framework with
Vienna Ab-initio Simulation Package (VASP)52,53. The core level
electron wave functions were expanded with the full-potential
projected augmented wave (PAW) method54 and the generalized
gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE)
exchange-correlation functional was used to describe the interaction between electrons55. The cutoff energy for the plane wave
expansion was set to 400 eV. The convergence criteria for the forceof the structure and the energy of the self-consistent calculation
were set at 10−2 eV Å−1 and 10−5 eV, respectively. A vacuum layer of
30 Å was built for all structures to avoid perturbation between
neighboring layers. The transitional state was searched and theenergy barrier was calculated using the CI-NEB56 with a force
threshold of 0.03 eV Å−1. The Brillion zone was sampled by a 3 × 3 × 1
grid mesh. The molecular dynamic simulation was achieved by theab-initio molecular dynamic (AIMD) calculation with VASP. Considering the calculation efﬁciency, the energy cutoff was reduced to
350 eV and the Brillouin zone was only sampled by Γ point. AIMD
trajectories were run with the canonical ensemble (NVT) at thetemperature of 573 K. The time step of the AIMD was 2 fs with an
overall time span of 20 and 15 ps for interdiffusion and crystal
growth, respectively. The central part of Se1-Bi1-Se2-Bi01 -Se01 was ﬁxed
to simulate the primary cluster during the simulation.

Device measurement

Modeling and COMSOL simulation

The temporal photovoltage and electrical performance of thedevice were recorded by the Agilent 4155 C semiconductor analyzer
with a standard electrical probe station. The lasers (405, 450, 532, 635,
and 1550 nm) were guided by an optical ﬁber and focused on thedevice with a microscope system, and were calibrated by a power
meter (PM400, Thorlabs). For 405, 450, 532, and 635 nm lasers, ahomemade light chopper and an attenuator were used to control thelight and its intensity, respectively. While for 1550 nm laser, a modulator (AFG 2005, Arbitrary Function Generator, Good Will Instrument
Co. Ltd) attached to the laser generator was used to modulate the IR
irradiation.
The detectivity (D*) was extracted as:

Two substrates were constructed with PI and SiO2. The silicon wafer
used in this work was covered with 285 nm SiO2. Thus, the bottom Si
was omitted from the model. The ﬁnite element method (FEM) was
carried out via COMSOL Multiphysics. The electrode was built on
the substrate based on the actual size of the device. In the meantime, the PTE ﬁlm on the substrates was only tens of nanometers,
which indicated that the thermal distribution of the substrate
dominates that of the ﬁlm. Hence, we neglected the effect of the ﬁlm
on the heat transfer in the COMSOL simulation. For the boundary
condition, a heat transfer coefﬁcient of convection of 5 W m−2 K−1
was used and applied to all top surfaces, and the bottom surface ofthe simulation domain was ﬁxed at room temperature (293 K). The
density (SiO2: 2200 kg m−3; PI: 1300 kg m−3), heat capacity (SiO2:
730 J kg−1 K−1; PI: 1100 J kg−1 K−1), and thermal conductivity (SiO2:
1.4 W m−1 K−1; PI: 0.15 W m−1 K−1) were extracted from the material
database in the software. Furthermore, the surface emissivity wasconsidered to calculate the temperature distribution (0.83 and 0.75
for PI and SiO2, respectively). The deposited beam power density P
was quantiﬁed by Gaussian distribution as:

D* =pﬃﬃﬃﬃﬃﬃﬃﬃﬃ
AΔf
NEP

ð3Þ

where A is the device area, and Δf is the bandwidth. The NEP was
calculated by NEP = vn/R, where vn is the measured voltage noise(Supplementary Fig. 22), and R is the responsivity. To determine theresponse time, the high-resolution photovoltage curve was recorded
by a highly sensitive source/measurement system (B2912A, Keysight).
Rise and decay times are deﬁned as the time interval of the net voltagechange between 10% and 90%, respectively. The temperature changeon the ﬁlm was recorded by an infrared camera (Ti480 PRO, Fluke).
Speciﬁcally, this test was carried out with a larger beam size (~1.5 mm)
to ensure it exceeded the minimum resolution of the infrared camera.
Moreover, the measurement was synchronized with the change ofphotovoltage with and without illumination.

Before scanning, a light power meter was placed directly below thesample to test the actual light power applied to the device (3.4 μW).
The photovoltage was monitored by a source/measure unit (2636B,
Keithley).

Density functional theory (DFT) calculation2

1d
exp  2
P=
2r
2πr 2

!
ð4Þ

where r is the radius of the laser, and d is the distance between thelight spot center and the speciﬁed point. The laser radius was 20 μm
and the total power of the deposited light beam was 0.5 mW in thesimulation.

Data availability
SPVM measurement
To investigate the working mechanism of the PTE detector, we characterized the photovoltage mapping by an integrated optoelectronic
scanning system (ScanPro Advance, Metatest). The spot size is controlled to be 3 μm to ensure a high spatial resolution of the mapping.
Nature Communications | (2024)15:728

Relevant data that support the key ﬁndings of this study are available
within the article and the Supplementary Information ﬁle. All raw data
generated during the current study are available from the corresponding author upon request. Source data are provided withthis paper.
7Article

References
1.

Li, J. et al. Wafer-scale single-crystal monolayer graphene grown on
sapphire substrate. Nat. Mater. 21, 740–747 (2022).
2. Zhao, C. et al. Evaporated tellurium thin ﬁlms for p-type ﬁeld-effecttransistors and circuits. Nat. Nanotechnol. 15, 53–58 (2020).
3. Wu, Z. et al. Large-scale growth of few-layer two-dimensional black
phosphorus. Nat. Mater. 20, 1203–1209 (2021).
4. Nomura, K. et al. Thin-ﬁlm transistor fabricated in single-crystallinetransparent oxide semiconductor. Science 300, 1269–1272 (2003).
5. Kim, M. G., Kanatzidis, M. G., Facchetti, A. & Marks, T. J. Lowtemperature fabrication of high-performance metal oxide thin-ﬁlm
electronics via combustion processing. Nat. Mater. 10,
382–388 (2011).
6. Park, S. et al. Low-temperature thin-ﬁlm deposition and crystallization. Science 297, 65 (2002).
7. Zhao, C. et al. Tellurium single-crystal arrays by lowtemperature evaporation and crystallization. Adv. Mater. 33,
2100860 (2021).
8. Park, S. et al. Self-powered ultra-ﬂexible electronics via nanograting-patterned organic photovoltaics. Nature 561,
516–521 (2018).
9. Shulaker, M. M. et al. Three-dimensional integration of nanotechnologies for computing and data storage on a single chip.
Nature 547, 74–78 (2017).
10. Jinno, H. et al. Stretchable and waterproof elastomer-coatedorganic photovoltaics for washable electronic textile applications.
Nat. Energy 2, 780–785 (2017).
11. Queisser, H. J. & Haller, E. E. Defects in semiconductors: some fatal,some vital. Science 281, 945–950 (1998).
12. Lu, Y. et al. Controlling defects in continuous 2D GaS ﬁlms for highperformance wavelength-tunable UV-discriminating photodetectors. Adv. Mater. 32, 1906958 (2020).
13. Li, F. et al. Crystalline InGaZnO quaternary nanowires with superlattice structure for high-performance thin-ﬁlm transistors. NanoRes. 12, 1796–1803 (2019).
14. Xu, X., Chen, J., Zhou, J. & Li, B. Thermal conductivity of polymers
and their nanocomposites. Adv. Mater. 30, 1705544 (2018).
15. Khan, A. I. et al. Ultralow-switching current density multilevel
phase-change memory on a ﬂexible substrate. Science 373,
1243–1247 (2021).
16. Lu, X., Sun, L., Jiang, P. & Bao, X. Progress of photodetectors
based on the photothermoelectric effect. Adv. Mater. 31,
1902044 (2019).
17. Gabor, N. M. et al. Hot carrier-assisted intrinsic photoresponse ingraphene. Science 334, 648–652 (2011).
18. Cai, X. et al. Sensitive room-temperature terahertz detection via thephotothermoelectric effect in graphene. Nat. Nanotechnol. 9,
814–819 (2014).
19. Viti, L. et al. Black phosphorus terahertz photodetectors. Adv. Mater.
27, 5567–5572 (2015).
20. Mauser, K. W. et al. Resonant thermoelectric nanophotonics. Nat.
Nanotechnol. 12, 770–775 (2017).
21. Badioli, M. et al. Phonon-mediated mid-infrared photoresponse ofgraphene. Nano Lett. 14, 6374–6381 (2014).
22. He, X. et al. Photothermoelectric p-n junction photodetector withintrinsic broadband polarimetry based on macroscopic carbon
nanotube ﬁlms. ACS Nano 7, 7271–7277 (2013).
23. Fan, P. et al. High-performance bismuth telluride thermoelectricthin ﬁlms fabricated by using the two-step single-source thermal
evaporation. J. Alloys Compd. 819, 153027 (2020).
24. Kim, T. H. et al. Tuning the Fermi level with topological phasetransition by internal strain in a topological insulator Bi2Se3 thin ﬁlm.
Nanoscale 8, 741–751 (2016).
25. Kong, D. et al. Rapid surface oxidation as a source of surface
degradation factor for Bi2Se3. ACS Nano 5, 4698–4703 (2011).

Nature Communications | (2024)15:728

https://doi.org/10.1038/s41467-024-44970-4
26. Su, X. et al. Self-propagating high-temperature synthesis for compound thermoelectrics and new criterion for combustion processing. Nat. Commun. 5, 4908 (2014).
27. Zheng, G. et al. High thermoelectric performance of mechanicallyrobust n-type Bi2Te3−xSex prepared by combustion synthesis. J.
Mater. Chem. A 3, 6603–6613 (2015).
28. Fu, J. et al. Understanding the combustion process for the synthesisof mechanically robust SnSe thermoelectrics. Nano Energy 44,
53–62 (2018).
29. Liang, T. et al. Ultra-fast non-equilibrium synthesis and phase segregation in InxSn1−xTe thermoelectrics by SHS-PAS processing. J.
Mater. Chem. C 3, 8550–8558 (2015).
30. Tan, X. et al. New criteria for the applicability of combustion
synthesis: the investigation of thermodynamic and kinetic processes for binary Chemical Reactions. J. Alloys Compd. 860,
158465 (2021).
31. Kundu, S. N., Bhattacharyya, D., Chaudhuri, S. & Pal, A. K. Synthesisof CuInSe2 ﬁlms by rapid thermal processing of stacked elemental
layers. Mater. Chem. Phys. 57, 207–213 (1999).
32. Dai, M. et al. Long-wave infrared photothermoelectric detectors
with ultrahigh polarization sensitivity. Nat. Commun. 14,3421 (2023).
33. Lu, X., Jiang, P. & Bao, X. Phonon-enhanced photothermoelectriceffect in SrTiO3 ultra-broadband photodetector. Nat. Commun. 10,
138 (2019).
34. Jin, Q. et al. Flexible layer-structured Bi2Te3 thermoelectric on acarbon nanotube scaffold. Nat. Mater. 18, 62–68 (2019).
35. Yang, Q. et al. Flexible thermoelectrics based on ductile semiconductors. Science 377, 854–858 (2022).
36. Kim, S. J. et al. High-performance ﬂexible thermoelectric power
generator using laser multiscanning lift-off process. ACS Nano 10,10851–10857 (2016).
37. Buscema, M. et al. Large and tunable photothermoelectric effect insingle-layer MoS2. Nano Lett. 13, 358–363 (2013).
38. Liu, Y. et al. High-performance ultrabroadband photodetector
based on photothermoelectric effect. ACS Appl. Mater. Interfaces
14, 29077–29086 (2022).
39. Wang, R. et al. Manipulating nanowire structures for an enhanced
broad-band ﬂexible photothermoelectric photodetector. Nano Lett.
22, 5929–5935 (2022).
40. Yan, Y. et al. Topological surface state enhanced photothermoelectric effect in Bi2Se3 nanoribbons. Nano Lett. 14,
4389–4394 (2014).
41. Echtermeyer, T. J. et al. Photothermoelectric and photoelectric
contributions to light detection in metal-graphene-metal photodetectors. Nano Lett. 14, 3733–3742 (2014).
42. Lv, B. et al. Local large temperature difference and ultra-wideband
photothermoelectric response of the silver nanostructure ﬁlm/
carbon nanotube ﬁlm heterostructure. Nat. Commun. 13,
1835 (2022).
43. Zhang, M., Ban, D., Xu, C. & Yeow, J. T. W. Large-area and broadband
thermoelectric infrared detection in a carbon nanotube black-bodyabsorber. ACS Nano 13, 13285–13292 (2019).
44. Dai, M. et al. High-performance, polarization-sensitive, long-wave
infrared photodetection via photothermoelectric effect withasymmetric van der Waals contacts. ACS Nano 16, 295–305 (2022).
45. Li, G. et al. Fast photothermoelectric response in CVD-grown PdSe2
photodetectors with in-plane anisotropy. Adv. Funct. Mater. 31,
2104787 (2021).
46. Saeed, Y., Singh, N. & Schwingenschlögl, U. Enhanced thermoelectric ﬁgure of merit in strained Tl-doped Bi2Se3. Appl. Phys. Lett.
105, 031915 (2014).
47. Guo, W. et al. Sensitive terahertz detection and imaging driven by
the photothermoelectric effect in ultrashort-channel black phosphorus devices. Adv. Sci. 7, 1902699 (2020).

8

Article
48. Mashhadi, S., Duong, D. L., Burghard, M. & Kern, K. Efﬁcient photothermoelectric conversion in lateral topological insulator heterojunctions. Nano Lett. 17, 214–219 (2016).
49. Wu, D. et al. Plasmon-enhanced photothermoelectric conversion inchemical vapor deposited graphene p-n junctions. J. Am. Chem.
Soc. 135, 10926–10929 (2013).
50. Wei, J. et al. Mid-infrared semimetal polarization detectors with
conﬁgurable polarity transition. Nat. Photonics 15, 614–621 (2021).
51. Shu, Y. et al. Modiﬁcation of bulk heterojunction and Cl doping forhigh-performance thermoelectric SnSe2/SnSe nanocomposites.
ACS Appl. Mater. Interfaces 10, 15793–15802 (2018).
52. Kresse, G. & Furthmüller, J. Efﬁciency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis
set. Comp. Mater. Sci. 6, 15–50 (1996).
53. Kresse, G. & Furthmüller, J. Efﬁcient iterative schemes for ab initio
total-energy calculations using a plane-wave basis set. Phys. Rev. B
54, 11169–11186 (1996).
54. Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 50,
17953–17979 (1994).
55. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradientapproximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996).
56. Henkelman, G., Uberuaga, B. P. & Jónsson, H. A climbing image
nudged elastic band method for ﬁnding saddle points and minimum energy paths. J. Chem. Phys. 113, 9901–9904 (2000).

Acknowledgements
This research was ﬁnancially supported by a fellowship award from the
Research Grants Council of the Hong Kong Special Administrative
Region, China (CityU RFS2021−1S04) and the State Key Laboratory of
Terahertz and Millimeter Waves (City University of Hong Kong).

Author contributions
Y.Z., Y.M., and J.C.H. conceived the study and designed the experiments. Y.Z. and Y.M. carried out the sample preparation, XRD characterization, and device fabrication. L.W. and Y.Lu performed the TEM
characterization. C.L. contributed to the SPVM measurement. Q.Q.
contributed to the temperature measurement via the infrared camera.
W.W. contributed to the XPS test. Z.L. performed the AFM and SEM
measurements. Y.Li performed the DSC measurement. W.W., W.J.W.,
D.L., P.X. helped with the electrical measurement. Y.Z. performed the

Nature Communications | (2024)15:728

https://doi.org/10.1038/s41467-024-44970-4
DFT calculation and COMSOL simulation. D.Y., D.C., Z.Y., S.P.Y., andJ.C.H. contributed to the data analysis. Y.Z., Y.M., Y.Lu, C.Y.W., and J.C.H
wrote and revised the manuscript.

Competing interests
The authors declare no competing interests.

Additional information
Supplementary information The online version contains
supplementary material available athttps://doi.org/10.1038/s41467-024-44970-4.
Correspondence and requests for materials should be addressed to You
Meng, Chun-Yuen Wong or Johnny C. Ho.
Peer review information Nature Communications thanks Chongwu
Wang and the other, anonymous, reviewer for their contribution to thepeer review of this work. A peer review ﬁle is available.
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