Query: Antenna-coupled photon emission from hexagonal boron nitride tunnel junctions

Candidate papers:

abstract: The ultrafast conversion of electrical signals to optical signals at the nanoscale is of fundamental interest for data processing, telecommunication and optical interconnects. However, the modulation bandwidths of semiconductor light emitting diodes are limited by the spontaneous recombination rate of electron hole pairs, and the footprint of electrically driven ultrafast lasers is too large for practical on chip integration. A metal insulator metal tunnel junction approaches the ultimate size limit of electronic devices and its operating speed is fundamentally limited only by the tunnelling time. Here, we study the conversion of electrons (localized in vertical gold hexagonal boron nitride gold tunnel junctions) to free space photons, mediated by resonant slot antennas. Optical antennas efficiently bridge the size mismatch between nanoscale volumes and far field radiation and strongly enhance the electron photon conversion efficiency. We achieve polarized, directional and resonantly enhanced light emission from inelastic electron tunnelling and establish a novel platform for studying the interaction of electrons with strongly localized electromagnetic fields.
author_names: Markus Parzefall

Palash Bharadwaj

A Jain

Takashi Taniguchi

K Watanabe

Lukas Novotny
title: Antenna coupled photon emission from hexagonal boron nitride tunnel junctions.
venue: Nature nanotechnology
year: 2015

abstract: We show that ordered monolayers of organic molecules stabilized by hydrogen bonding on the surface of exfoliated few layer hexagonal boron nitride (hBN) flakes may be incorporated into van der Waals heterostructures with integral few layer graphene contacts forming a molecu lar/2D hybrid tunneling diode. Electrons can tunnel from through the hBN/molecular barrier under an applied voltage VSD and we observe molecular electroluminescence from an excited singlet state with an emitted photon energy h eVSD, indicating up conversion by energies up to 1 eV. We show that tunnelling electrons excite emebedded molecules into singlet states in a two step process via an intermediate triplet state through inelastic scattering and also ob serve direct emission from the triplet state. These heterostructures provide a solid state device in which spin triplet states can be controllably excited and provide a new route to investigate the physics, chemistry and quantum spin based applications of triplet generation, emission and molecular photon up conversion.
author_names: Simon A Svatek

James R Kerfoot

Alex Summerfield

Anton S Nizovtsev

Vladimir V Korolkov

Takashi Taniguchi

Kenji Watanabe

Elisa Antol'in

Elena Besley

Peter H Beton
title: Triplet excitation and electroluminescence from a supramolecular monolayer embedded in a boron nitride tunnel barrier.
venue: Nano letters
year: 2019

abstract: We show that ordered monolayers of organic molecules stabilized by hydrogen bonding on the surface of exfoliated few layer hexagonal boron nitride (hBN) flakes may be incorporated into van der Waals heterostructures with integral few layer graphene contacts forming a molecular/2D hybrid tunneling diode. Electrons can tunnel from through the hBN/molecular barrier under an applied voltage VSD and we observe molecular electroluminescence from an excited singlet state with an emitted photon energy eVSD, indicating up conversion by energies up to 1 eV. We show that tunnelling electrons excite embedded molecules into singlet states in a two step process via an intermediate triplet state through inelastic scattering and also observe direct emission from the triplet state. These heterostructures provide a solid state device in which spin triplet states, which cannot be generated by optical transitions, can be controllably excited and provide a new route to investigate the physics, chemistry and quantum spin based applications of triplet generation, emission and molecular photon up conversion.
author_names: Simon A Svatek

James R Kerfoot

Alex Summerfield

Anton S Nizovtsev

Vladimir V Korolkov

Takashi Taniguchi

Kenji Watanabe

Elisa Antolin

Elena Besley

Peter H Beton
title: Triplet excitation and electroluminescence from a supramolecular monolayer embedded in a boron nitride tunnel barrier
year: 2019

abstract: We report on efficient carrier to exciton conversion and planar electroluminescence from tunnel diodes based on a metal insulator semiconductor (MIS) van der Waals heterostack consisting of few layer graphene (FLG) hexagonal boron nitride (hBN) and monolayer tungsten disulfide (WS2) These devices exhibit excitonic electroluminescence with extremely low threshold current density of a few pA*mm 2, which is several orders of magnitude lower compared to the previously reported values for the best planar EL devices. Using a reference dye, we estimate the EL quantum efficiency to be ~1% at low current density limit, which is of the same order of magnitude as photoluminescence quantum yield at the equivalent excitation rate. Our observations reveal that the efficiency of our devices is not limited by carrier to exciton conversion efficiency but by the inherent exciton to photon yield of the material. The device characteristics indicate that the light emission is triggered by injection of hot minority carriers (holes) to n doped WS2 by Fowler Nordheim tunneling and that hBN serves as an efficient hole transport and electron blocking layer. Our findings offer insight into the intelligent design of van der Waals heterostructures and avenues for realizing efficient excitonic devices.
author_names: Shunfeng Wang

Junyong Wang

Weijie Zhao

Francesco Giustiniano

Leiqiang Chu

Ivan A Verzhbitskiy

Justin Zhou Yong

Goki Eda
title: Efficient Carrier to Exciton Conversion in Field Emission Tunnel Diodes Based on MIS Type van der Waals Heterostack.
venue: Nano letters
year: 2017

abstract: Layered materials (LM) and their heterostructures (LMH) provide a promising platform for applications in photonics and electronics [1] such as tunneling junctions [2] photodetectors [3] and light emitting diodes (LEDs) [4] LEDs based on LM exploit the direct bandgap of semiconducting monolayer transition metal dichalcogenides (1L TMDs) [5] A key parameter to define the performances of LEDs is the external quantum efficiency (EQE) i.e. the ratio between emitted photons and injected electrons. Thus far, the EQE in LEDs based on LMH reached values ~0.2%[6] Here, we fabricate LEDs via vertically stacking singlelayer graphene (SLG) used as a transparent electrode for charge injection, few layer hexagonal boron nitride (hBN) used as a tunnel barrier, and a 1L TMDs (MoS2, WS2, WSe2) for light emission (Fig. 1) We measure the tunneling current, Fig. 2a, and electroluminescence, Fig. 2b. We show that a chemical treatment using the superacid bis(trifluoromethane)sulfonimide (TFSI) enhances by more than four times the EQE in LMH based LEDs, up to 1% (Fig. 2) We present simulations and modeling of integration of LMH LEDs with photonic cavities for further enhancement of EQE. This approach paves the way to highly efficient LEDs. References
author_names: Domenico De Fazio

Ioannis Paradisanos

Takashi Taniguchi

Giancarlo Soavi

Elefterios Lidorikis

Ilya Goykhman

Andrea C Ferrari
title: Graphene 2019 June 25 28 2019 Rome Italy Light Emitting Diodes Based on Layered Heterostructures on Photonic Cavities
year: 2019

abstract: The understanding of and control over light emission from quantum tunneling has challenged researchers for more than four decades due to the intricate interplay of electrical and optical properties in atomic scale volumes. Here we introduce a device architecture that allows for the disentanglement of electronic and photonic pathways van der Waals quantum tunneling devices. The electronic properties are defined by a stack of two dimensional atomic crystals whereas the optical properties are controlled via an external photonic architecture. In van der Waals heterostructures made of gold, hexagonal boron nitride and graphene we find that inelastic tunneling results in the emission of photons and surface plasmon polaritons. By coupling these heterostructures to optical nanocube antennas we achieve resonant enhancement of the photon emission rate in narrow frequency bands by four orders of magnitude. Our results lead the way towards a new generation of nanophotonic devices that are driven by quantum tunneling.Electrical and optical properties of light emitting devices driven by quantum tunneling are closely intertwined. Parzefall et al. show that these properties can be individually controlled by cointegrating van der Waals heterostructures with optical antennas.
author_names: Markus Parzefall

Aron Szabo

Takashi Taniguchi

Kenji Watanabe

Mathieu Luisier

Lukas Novotny
title: Light from van der Waals quantum tunneling devices
venue: Nature Communications
year: 2019

abstract: Two dimensional (2 D) hexagonal boron nitride (h BN) has attracted considerable attention for deep ultraviolet optoelectronics and visible single photon sources, however, realization of an electrically driven light emitter remains challenging due to its wide bandgap nature. Here, we report electrically driven visible light emission with a red shift under increasing electric field from a few layer h BN by employing a five period Al2O3/h BN multiple heterostructure and a graphene top electrode. Investigation of electrical properties reveals that the Al2O3 layers act as potential barriers confining injected carriers within the h BN wells, while suppressing the electrostatic breakdown by trap assisted tunneling, to increase the probability of radiative recombination. The result highlights a promising potential of such multiple heterostructure as a practical and efficient platform for electrically driven light emitters based on wide bandgap two dimensional materials.
author_names: Seunghee Lee

Hokyeong Jeong

Dong Yeong Kim

Seung-Young Seo

Cheolhee Han

Odongo Francis Ngome Okello

J I Lo

Yu-Chain Peng

Chan-Hyoung Oh

Gyeong Won Lee

Jong-In Shim

Bing-Ming Cheng

Kyung Song

Sihyuk Choi

Moon-Ho Jo

Jong Kyu Kim
title: Electroluminescence from h BN by using Al2O3/h BN multiple heterostructure.
venue: Optics express
year: 2019

abstract: Single defects in monolayer WSe2 have been shown to be a new class of single photon emitters and have potential applications in quantum technologies. Whereas previous work relied on optical excitation of single defects in isolated WSe2 monolayers, in this work we demonstrate electrically driven single defect light emission by using both vertical and lateral van der Waals heterostructure devices. In both device geometries, we use few layer graphene as the source and drain and hexagonal boron nitride as the dielectric spacer layers for engineered tunneling contacts. In addition, the lateral devices utilize a split back gate design to realize an electrostatically defined p i n junction. At low current densities and low temperatures ~5 K) we observe narrow spectral lines in the electroluminescence (EL) whose properties are consistent with optically excited defect bound excitons. We show that the emission originates from spatially localized regions of the sample, and the EL spectrum from single defects has a doublet with the characteristic exchange splitting and linearly polarized selection rules. All are consistent with previously reported single photon emitters in optical measurements. Our results pave the way for on chip and electrically driven single photon sources in two dimensional semiconductors for quantum technology applications.
author_names: Genevieve Clark

John R Schaibley

Jason S Ross

Takashi Taniguchi

Kenji Watanabe

Joshua R Hendrickson

Shin Mou

Wang Yao

Xiaodong Xu
title: Single Defect Light Emitting Diode in a van der Waals Heterostructure.
venue: Nano letters
year: 2016

abstract: Description
author_names: Tiancheng Song

Eric Anderson

Matisse Wei-Yuan Tu

Kyle L Seyler

Takashi Taniguchi

Kenji Watanabe

Michael A McGuire

Xiaosong Li

Ting Cao

Di Xiao

Wang Yao

Xiaodong Xu
title: Spin photovoltaic effect in magnetic van der Waals heterostructures
venue: Science advances
year: 2021

abstract: Light emitting diodes are of importance for lighting, displays, optical interconnects, logic and sensors. Hence the development of new systems that allow improvements in their efficiency, spectral properties, compactness and integrability could have significant ramifications. Monolayer transition metal dichalcogenides have recently emerged as interesting candidates for optoelectronic applications due to their unique optical properties. Electroluminescence has already been observed from monolayer MoS2 devices. However, the electroluminescence efficiency was low and the linewidth broad due both to the poor optical quality of MoS2 and to ineffective contacts. Here, we report electroluminescence from lateral p n junctions in monolayer WSe2 induced electrostatically using a thin boron nitride support as a dielectric layer with multiple metal gates beneath. This structure allows effective injection of electrons and holes, and combined with the high optical quality of WSe2 it yields bright electroluminescence with 1000 times smaller injection current and 10 times smaller linewidth than in MoS2. Furthermore, by increasing the injection bias we can tune the electroluminescence between regimes of impurity bound, charged, and neutral excitons. This system has the required ingredients for new kinds of optoelectronic devices such as spinand valley polarized light emitting diodes, on chip lasers, and two dimensional electro optic modulators. Main Text Few layer group VIB transition metal dichalcogenides (TMDs) represent a class of semiconductors in the two dimensional (2D) limit. Due to their large carrier effective mass and the reduced screening in 2D, electron hole interactions are much stronger than in conventional semiconductors. This leads to large binding energies for both charged and neutral excitons which as a result are spectrally sharp, robust, and amenable to electrical manipulation. In addition, the large spin orbit coupling and the acentric structure of TMDs provides a connection between spin and valley degrees of freedom, light polarization, and magnetic and electric fields that can be exploited for new kinds of device operation. Although in bulk TMDs the band gap is indirect, in the limit of a single monolayer it becomes direct, fulfilling the most basic requirement for efficient light emission. Indeed, electroluminescence (EL) has already been reported from monolayer MoS2 field effect transistors (FETs) occurring near the Schottky contact with a metal or with highly doped silicon. However, the efficiency and spectral quality was much lower than has been demonstrated for other nanoscale light emitters such as carbon nanotubes, for two reasons. First, efficient EL requires effective injection of both electrons and holes into the active region, which should therefore be within a p n junction. Second, MoS2 is known to have poorer optical quality than other group VIB TMDs, possibly due to impurities. It has been previously shown that in contrast monolayer WSe2 has excellent optical properties. Here, we demonstrate that combining monolayer WSe2 with a p n junction architecture using electrostatic doping produces efficient and electrically tunable excitonic light emitting diodes (LEDs) An optical image and a schematic of a device, made by a combination of electron beam lithography and transfer of exfoliated sheets, are shown Figs. 1a and 1b. A monolayer WSe2 sheet sits on a sheet of hexagonal boron nitride (BN) typically 10 nm thick, which acts both as a smooth, disorder free substrate to minimize non radiative energy relaxation pathways and as a high quality gate dielectric. Applying voltages to the two 7 nm palladium gate electrodes beneath the BN can create two separate electrostatically doped regions in the WSe2 separated by a 300 nm wide undoped strip. Gold/vanadium (60/6 nm) source and drain contacts are evaporated on top. Importantly, they overlap the gates in order to reduce the Schottky barriers at the contacts. For electrical transport measurements a dc bias is applied to one contact (the source) and the current from the other (the drain) is measured by a virtual earth current preamplifier. The silicon substrate is grounded. We start by showing that a p n junction can be created electrostatically. First we set the gate voltages Vg1 and Vg2 to the same value. Figure 1c shows the current produced by a bias V 0.5 V as the gate voltage Vg1 Vg2 is swept at 60 K (room temperature measurements are shown in Supplementary Figure 1) The current increases rapidly for gate voltages +6.5 V (electron doping) and 6.5 V (hole doping) demonstrating ambipolar operation. In the inset the red I V curve, taken at Vg1 Vg2 +8.0 V, is almost symmetric as expected for both gated regions being equally electron doped. The nonlinearity near zero bias can be associated with the undoped gap between the gates and residual Schottky barriers at the contacts. In contrast, the blue I V curve, taken with Vg1 +8.0 V and Vg2 8.0 V, shows the strong rectification behavior expected for a p n junction. The p n junction can be investigated in detail by scanning optical measurements. Fig. 2a is a microscope image of a device and Fig. 2b is a corresponding scanning photocurrent image, measured with zero bias at 100 K using a 10 mW diffraction limited 660 nm laser spot scanned over the sample. We see a large photocurrent signal localized between the gates, with a peak magnitude of 5 nA. Taking into account the 1% absorption of WSe2 monolayers at 660 nm, the internal quantum efficiency reaches a maximum of about 5% Such a photocurrent is the natural result of the junction functioning as a photodiode, with photogenerated carriers separated by a strong depletion field concentrated in the undoped gap. The sensitivity of the photodiode can be tuned over a wide range by varying the gate voltages and bias (see Supplemental Figure 2) Fig. 2c is a corresponding map of the integrated photoluminescence (PL) intensity, which indicates the high optical quality of the whole WSe2 sheet and shows that the luminescence is not substantially quenched by the underlying gates. More revealingly, Fig. 2d shows a colour map of the peak PL photon energy, exhibiting two distinct regions clearly correlated with the expected ndoped (blue) and p doped (red) parts of the WSe2 above the gates. The reason for this is made clear in Fig. 2e, which shows PL spectra taken at different positions. The detailed origin of these spectral features has been established previously. Above the gate held at Vg1 +8.0 V (blue trace) the negatively charged X trion (two electrons and one hole) dominates, implying an excess of electrons. Above the other gate, held at Vg2 8.0 V (red trace) the higher energy positively charged X trion (two holes and one electron) dominates, implying an excess of holes. In the gap between the gates (black trace) the neutral exciton X peak can also be seen, consistent with no doping in that region. Here the superimposed X and X peaks may come from the gated regions, since the laser spot is larger than the gap. When the device is configured as a p n junction (Vg1 Vg2 8 V) but not otherwise, we observe bright electroluminescence. Good spectra can be obtained even at room temperature with a current of 5 nA, as illustrated (blue) in Fig. 3a. This current is three orders of magnitude smaller than in MoS2 FETs. In fact, in our best device we observed EL at an injection current as low as 200 pA (See Supplemental Figure 3) To understand the nature of the EL, we superimpose a normalized PL spectrum of undoped monolayer WSe2 (red) It is known that the PL of WSe2 is from the recombination of direct gap excitons; thus the similarity between the EL and PL spectra implies that the injected electrons and holes form excitons before recombining radiatively. This is a natural consequence of the large exciton binding energy due to the strong Coulomb interaction in monolayer TMDs. Fig. 3b shows an image of the total EL intensity (coloured) superimposed on a simple white light reflection image (grayscale) It is clear that the EL emanates from the entire length of the monolayer junction between the two gates. At low temperatures the EL spectrum develops interesting structure. Fig. 4a shows a plot of EL intensity as a function of current and photon energy. There are three main spectral features: a narrow higher energy peak (green arrow) a broad central peak (brown arrow) and a lower energy peak (black arrow) The shapes and relative intensities of these features change with current. Their origins can be deduced from a comparison with the PL, whose intensity is plotted in Fig. 4b as a function of photon energy and common gate voltage Vg=Vg1=Vg2. Here we see the tuning of the dominant exciton species as the carrier density is changed by gating. The PL feature which is strongest at Vg 0 is due to X recombination. It has a similar width to, and is at the same position (1.69 eV) as, the narrow EL peak, which we therefore identify as the X emission. The dominant PL feature at Vg 0, which shifts from 1.663 eV to 1.625 eV as Vg increases from 0 to +8 V, is due to X (of which there are multiple species) The broad EL feature occurs in the same range of energies, implying that it is dominated by X. The dominant feature in the PL at Vg 0 is due to X. This aligns with the high energy shoulder (grey arrow) on the broad EL feature at about 1.670 eV. There is also a band of emission from impurity bound excitons (X) in the PL which matches the lower energy EL feature centered at 1.59 eV. The finding that X dominates the EL in Fig. 4a is consistent with the observation that X has much stronger PL than the other exciton species (Fig. 4b) The shifts of the trion peaks in the PL with Vg imply that the trion binding energy depends on perpendicular electric field. Hence the fact that the width of the broad peak in the EL matches the full range of X energies in the P
author_names: Jason S Ross

Philip Klement

Aaron M Jones

Nirmal J Ghimire

David G Mandrus

Takashi Taniguchi

Kenji Watanabe

Kenji Kitamura

Wang Yao

David H Cobden

Xiaodong Xu
title: Title: Electrically Tunable Excitonic Light Emitting Diodes based on Monolayer WSe2 p n Junctions
year: 2014