ORCID Profile
0000-0003-3701-8119
Current Organisation
National Institute for Materials Science
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Probability Theory | Stochastic Analysis and Modelling | Statistics |
Publisher: IOP Publishing
Date: 16-02-2023
Abstract: Excitons in thin layers of semiconducting transition metal dichalcogenides are highly subject to the strongly modified Coulomb electron-hole interaction in these materials. Therefore, they do not follow the model system of a two-dimensional hydrogen atom. We investigate experimentally and theoretically excitonic properties in both the monolayer (ML) and the bilayer (BL) of MoSe 2 encapsulated in hexagonal BN. The measured magnetic field evolutions of the reflectance contrast spectra of the MoSe 2 ML and BL allow us to determine g -factors of intralayer A and B excitons, as well as the g -factors of the interlayer exciton. We explain the dependence of g -factors on the number of layers and excitation state can be explained using first principles calculations. Furthermore, we demonstrate that the experimentally measured ladder of excitonic s states in the ML can be reproduced using the k·p approach with the Rytova-Keldysh potential that describes the electron-hole interaction. In contrast, the analogous calculation for the BL case require taking into account the out-of-plane dielectric response of the MoSe 2 BL.
Publisher: American Physical Society (APS)
Date: 08-12-2022
Publisher: Royal Society of Chemistry (RSC)
Date: 2015
DOI: 10.1039/C5CP00532A
Abstract: Boron nitride nanosheets covered by gold nanoparticles are controllably fabricated for highly-sensitive and reusable substrates for surface enhanced Raman spectroscopy.
Publisher: American Chemical Society (ACS)
Date: 11-08-2023
Publisher: American Chemical Society (ACS)
Date: 18-05-2022
Publisher: AIP Publishing
Date: 18-04-2022
DOI: 10.1063/5.0085950
Abstract: The dielectric environment of atomically thin monolayer (ML) of semiconducting transition metal dichalcogenides affects both the electronic bandgap and the excitonic binding energy in the ML. We investigate the effect of the environment on the in-plane magnetic field brightening of neutral and charged dark exciton emissions in the WSe2 ML. The monolayers placed in three dielectric environments are studied, in particular, the ML encapsulated in hexagonal BN (hBN) flakes, the ML deposited on a hBN layer, and the ML embedded between the hBN flake and SiO2/Si substrate. We observe that the brightening rates of the neutral and charged dark excitons depend on the dielectric environment, which may be related to the variation of the level of carrier concentration in the ML. Moreover, the surrounding media, characterized by different dielectric constants, weakly influence the relative energies of the neutral and charged dark excitons in reference to the bright ones.
Publisher: Springer Science and Business Media LLC
Date: 16-08-2021
DOI: 10.1038/S41467-021-24925-9
Abstract: Engineering non-linear hybrid light-matter states in tailored lattices is a central research strategy for the simulation of complex Hamiltonians. Excitons in atomically thin crystals are an ideal active medium for such purposes, since they couple strongly with light and bear the potential to harness giant non-linearities and interactions while presenting a simple s le-processing and room temperature operability. We demonstrate lattice polaritons, based on an open, high-quality optical cavity, with an imprinted photonic lattice strongly coupled to excitons in a WS 2 monolayer. We experimentally observe the emergence of the canonical band-structure of particles in a one-dimensional lattice at room temperature, and demonstrate frequency reconfigurability over a spectral window exceeding 85 meV, as well as the systematic variation of the nearest-neighbour coupling, reflected by a tunability in the bandwidth of the p-band polaritons by 7 meV. The technology presented in this work is a critical demonstration towards reconfigurable photonic emulators operated with non-linear photonic fluids, offering a simple experimental implementation and working at ambient conditions.
Publisher: IOP Publishing
Date: 29-06-2023
Abstract: Integration of graphene and hexagonal boron nitride (hBN) in lateral heterostructures has provided a route to broadly engineer the material properties by quantum confinement of electrons or introduction of novel electronic and magnetic states at the interface. In this work we demonstrate lateral heteroepitaxial growth of graphene nanoribbons (GNRs) passivated by hBN using high-temperature molecular beam epitaxy (HT-MBE) to grow graphene in oriented hBN trenches formed ex-situ by catalytic nanoparticle etching. High-resolution atomic force microscopy (AFM) reveals that GNRs grow epitaxially from the etched hBN edges, and merge to form a GNR network passivated by hBN. Using conductive AFM we probe the nanoscale electrical properties of the nanoribbons and observe quasiparticle interference patterns caused by intervalley scattering at the graphene/hBN interface, which carries implications for the potential transport characteristics of hBN passivated GNR devices.
Publisher: Springer Science and Business Media LLC
Date: 23-05-2015
Publisher: Royal Society of Chemistry (RSC)
Date: 2020
DOI: 10.1039/D0CP02737E
Abstract: Magnetic field induced valley polarization of carriers is substantially different for the absorption and emission response of a WS 2 monolayer.
Publisher: American Chemical Society (ACS)
Date: 08-2013
DOI: 10.1021/NL4012529
Abstract: Kelvin probe microscopy in ultrahigh vacuum is used to image the local electrostatic potential fluctuations above hexagonal boron nitride (h-BN) and SiO2, common substrates for graphene. Results are compared to a model of randomly distributed charges in a two-dimensional (2D) plane. For SiO2, the results are well modeled by 2D charge densities ranging from 0.24 to 2.7 × 10(11) cm(-2), while h-BN displays potential fluctuations 1-2 orders of magnitude lower than SiO2, consistent with the improvement in charge carrier mobility for graphene on h-BN compared to SiO2. Electron beam exposure of SiO2 increases the charge density fluctuations, creating long-lived metastable charge populations of ~2 × 10(11) cm(-2) at room temperature, which can be reversed by heating.
Publisher: American Chemical Society (ACS)
Date: 05-12-2014
DOI: 10.1021/NL503411A
Abstract: Two-dimensional (2D) hexagonal boron nitride (BN) nanosheets are excellent dielectric substrate for graphene, molybdenum disulfide, and many other 2D nanomaterial-based electronic and photonic devices. To optimize the performance of these 2D devices, it is essential to understand the dielectric screening properties of BN nanosheets as a function of the thickness. Here, electric force microscopy along with theoretical calculations based on both state-of-the-art first-principles calculations with van der Waals interactions under consideration, and nonlinear Thomas-Fermi theory models are used to investigate the dielectric screening in high-quality BN nanosheets of different thicknesses. It is found that atomically thin BN nanosheets are less effective in electric field screening, but the screening capability of BN shows a relatively weak dependence on the layer thickness.
Publisher: AIP Publishing
Date: 24-07-2023
DOI: 10.1063/5.0155311
Abstract: Reliable methods to create quantum emitters in hexagonal boron nitride (hBN) are highly sought after for scalable applications in quantum photonic devices. Specifically, recent efforts have focused on defects in hBN with a zero phonon line at 2.8 eV (436 nm). Here, we employ carbon-doped hBN crystals that were irradiated by an electron beam to generate these emitters and perform annealing treatments to investigate the stability of the emitters. We find that the blue emitters are stable up to ∼800 °C. However, upon annealing to 1000 °C, the emitters disappear, and a family of other emitters appears in the region of hBN that had been irradiated by an electron beam. Our findings contribute to the understanding of emitter species and emitter formation in hBN.
Publisher: Springer Science and Business Media LLC
Date: 27-04-2015
Abstract: Atomically thin two-dimensional semiconductors such as MoS2 hold great promise for electrical, optical and mechanical devices and display novel physical phenomena. However, the electron mobility of mono- and few-layer MoS2 has so far been substantially below theoretically predicted limits, which has h ered efforts to observe its intrinsic quantum transport behaviours. Potential sources of disorder and scattering include defects such as sulphur vacancies in the MoS2 itself as well as extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, we have developed here a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Magneto-transport measurements show dramatic improvements in performance, including a record-high Hall mobility reaching 34,000 cm(2) V(-1) s(-1) for six-layer MoS2 at low temperature, confirming that low-temperature performance in previous studies was limited by extrinsic interfacial impurities rather than bulk defects in the MoS2. We also observed Shubnikov-de Haas oscillations in high-mobility monolayer and few-layer MoS2. Modelling of potential scattering sources and quantum lifetime analysis indicate that a combination of short-range and long-range interfacial scattering limits the low-temperature mobility of MoS2.
Publisher: AIP Publishing
Date: 09-04-2018
DOI: 10.1063/1.5026478
Abstract: We have combined spatially resolved steady-state micro-photoluminescence with time-resolved photoluminescence to investigate the exciton diffusion in a WSe2 monolayer encapsulated with hexagonal boron nitride. At 300 K, we extract an exciton diffusion length of LX = 0.36 ± 0.02 μm and an exciton diffusion coefficient of DX = 14.5 ± 2 cm2/s. This represents a nearly 10-fold increase in the effective mobility of excitons with respect to several previously reported values on nonencapsulated s les. At cryogenic temperatures, the high optical quality of these s les has allowed us to discriminate the diffusion of the different exciton species: bright and dark neutral excitons, as well as charged excitons. The longer lifetime of dark neutral excitons yields a larger diffusion length of LXD=1.5±0.02 μm.
Publisher: Royal Society of Chemistry (RSC)
Date: 2023
DOI: 10.1039/D2NR05985A
Abstract: The interfacial thermal conductance between graphene and atomically thin boron nitride has been experimentally measured for the first time.
Publisher: American Chemical Society (ACS)
Date: 07-09-2018
Abstract: The electrical contact to two-dimensional (2D) semiconductor materials is decisive to the electronic performance of 2D semiconductor field-effect devices (FEDs). The presence of a Schottky barrier often leads to a large contact resistance, which seriously limits the channel conductance and carrier mobility measured in a two-terminal geometry. In contrast, Ohmic contact is desirable and can be achieved by the presence of a nonrectifying or tunneling barrier. Here, we demonstrate that a nonrectifying barrier can be realized by contacting indium (In), a low work function metal, with layered InSe because of a favorable band alignment at the In-InSe interface. The nonrectifying barrier is manifested by Ohmic contact behavior at T = 2 K and a low barrier height, Φ
Publisher: American Physical Society (APS)
Date: 08-05-2020
Publisher: Royal Society of Chemistry (RSC)
Date: 2021
DOI: 10.1039/D1NR03855A
Abstract: Atomically thin materials, like semiconducting transition metal dichalcogenides, are highly sensitive to the environment.
Publisher: American Scientific Publishers
Date: 08-2013
Abstract: The hollow nanofibers of Mn-doped SnO2 were fabricated by electrospinning method. The structural and magnetic properties of the electrospun fibers calcined at 600 degrees C were studied. X-ray diffraction patterns of the nanofibers showed broad diffraction peaks and were indexed to the characteristic diffraction pattern of tetragonal SnO2. The hollow fiber micro-structure of Mn-doped and pure SnO2 were confirmed from the observed HRSEM and TEM analysis. Typical diameter of the hollow nanofibers was found to be around 150 nm. Strong emission peak in the visible region of the PL spectra characteristic of the optical activity of the SnO2 is obtained. Surface composition of the nanofiber and successful incorporation of Mn into SnO2 were confirmed from intense peaks recorded in the XPS spectra. Finally, a reasonable ferromagnetic transition observed at 10 K in the Mn-doped SnO2, substantiates that the presence of undetectable Sn-Mn solid solution or the formation of Mn based oxide secondary phases. It concludes that the induced ferromagnetism is only due to the precipitated impurity phases and does not arise from any intrinsic pure SnO2 or the dopant.
Publisher: Wiley
Date: 24-02-2021
Abstract: Due to Fermi level pinning (FLP), metal‐semiconductor contact interfaces result in a Schottky barrier height (SBH), which is usually difficult to tune. This makes it challenging to efficiently inject both electrons and holes using the same metal—an essential requirement for several applications, including light‐emitting devices and complementary logic. Interestingly, modulating the SBH in the Schottky–Mott limit of de‐pinned van der Waals (vdW) contacts becomes possible. However, accurate extraction of the SBH is essential to exploit such contacts to their full potential. In this study a simple technique is proposed to accurately estimate the SBH at the vdW contact interfaces by circumventing several ambiguities associated with SBH extraction. Using this technique on several vdW contacts, including metallic 2H‐TaSe 2 , semi‐metallic graphene, and degenerately doped semiconducting SnSe 2 , it is demonstrated that vdW contacts exhibit a universal de‐pinned nature. Superior ambipolar carrier injection properties of vdW contacts are demonstrated (with Au contact as a reference) in two applications, namely, a) pulsed electroluminescence from monolayer WS 2 using few‐layer graphene (FLG) contact, and b) efficient carrier injection to WS 2 and WSe 2 channels in both n‐type and p‐type field effect transistor modes using 2H‐TaSe 2 contact.
Publisher: American Chemical Society (ACS)
Date: 26-07-2023
Publisher: American Chemical Society (ACS)
Date: 22-10-2020
Publisher: Springer Science and Business Media LLC
Date: 14-09-2015
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.
Publisher: Springer Science and Business Media LLC
Date: 22-06-2017
DOI: 10.1038/NCOMMS15815
Abstract: Atomically thin boron nitride (BN) nanosheets are important two-dimensional nanomaterials with many unique properties distinct from those of graphene, but investigation into their mechanical properties remains incomplete. Here we report that high-quality single-crystalline mono- and few-layer BN nanosheets are one of the strongest electrically insulating materials. More intriguingly, few-layer BN shows mechanical behaviours quite different from those of few-layer graphene under indentation. In striking contrast to graphene, whose strength decreases by more than 30% when the number of layers increases from 1 to 8, the mechanical strength of BN nanosheets is not sensitive to increasing thickness. We attribute this difference to the distinct interlayer interactions and hence sliding tendencies in these two materials under indentation. The significantly better interlayer integrity of BN nanosheets makes them a more attractive candidate than graphene for several applications, for ex le, as mechanical reinforcements.
Publisher: Research Square Platform LLC
Date: 05-04-2023
DOI: 10.21203/RS.3.RS-2606377/V1
Abstract: Hexagonal boron nitride (hBN) has been a centre of interest due to its ability to host several bright quantum emitters at room temperature. However, the identification of the observed emitters remains challenging due to spectral variability as well as the lack of atomic defect structure information. In this work, we report two new blue quantum emitters with zero phonon line (ZPL) centred around 460 nm and 490 nm in hBN powders. We further demonstrate that the blue emissions can be created by high temperature annealing or high energy ion irradiation in exfoliated hBN flakes. Scanning transmission electron microscopy (STEM) reveals that the dominant defect structures present in ion irradiated s le are vacancy-type (V x ) and adatom(intercalant)-type (A x ). Together with first principle GW-BSE (Bethe-Salpeter equation) calculation, the observed blue emissions at 490 nm may be due to boron intercalants (B int ). Our results not only discover a new group of blue quantum emissions in hBN, but also provide an insight on the physical origin of the emissions by correlating the emission wavelength with local atomic structures in hBN.
Publisher: American Chemical Society (ACS)
Date: 03-05-2023
Publisher: Wiley
Date: 08-04-2023
Abstract: Monolayers (MLs) of transition‐metal dichalcogenides host efficient single‐photon emitters (SPEs) usually associated to the presence of nanoscale mechanical deformations or strain. Large‐scale spatial control of strain would enhance the scalability of such SPEs and allow for their incorporation into photonic structures. Here, the formation of regular arrays of strained hydrogen‐filled one‐layer‐thick micro‐domes obtained by H‐ion irradiation and lithography‐based approaches is reported. Typically, the H 2 liquefaction for temperatures T K causes the disappearance of the domes preventing their use as potential SPEs. Here, it is shown that the dome deflation can be overcome by hBN heterostructuring, that is by depositing thin hBN flakes on the domes. This leads to the preservation of the dome structure at all temperatures, as found by micro‐Raman and micro‐photoluminescence (µ‐PL) studies. Eventually, spatially controlled hBN‐capped WS 2 domes show the appearance, at 5 K, of intense emission lines originating from localized excitons, which are shown to behave as quantum emitters here. The electronic properties of the emitters are addressed by time‐resolved µ‐PL yielding time decays of 1–10 ns, and by magneto‐µ‐PL measurements. The latter provide an exciton magnetic moment a factor of two larger than the value observed in planar strain‐free MLs.
Publisher: IOP Publishing
Date: 19-12-2022
Abstract: The possibility of almost linear tuning of the band gap and of the electrical and optical properties in monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with on-demand characteristics. By making use of photoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited on SiO 2 /Si substrate and encapsulated in hexagonal BN flakes. Similarly to the ‘ parent ’ WS 2 and WSe 2 MLs, we assign the WSSe MLs to the ML family with the dark ground exciton state. We find that, in addition to the neutral bright A exciton line, three observed emission lines are associated with negatively charged excitons. The application of in-plane and out-of-plane magnetic fields allows us to assign undeniably the bright and dark (spin- and momentum-forbidden) negative trions as well as the phonon replica of the dark spin-forbidden complex. Furthermore, the existence of the single photon emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to enlarge the wavelength range for potential future quantum applications of S-TMDs.
Publisher: Research Square Platform LLC
Date: 07-10-2020
DOI: 10.21203/RS.3.RS-85675/V1
Abstract: In monolayer WSe 2 , interactions between the lower-energy momentum- and spin-indirect “dark” excitons and the bright exciton (X) are likely to be significant in determining the optical properties of X at high power, and limit the ultimate exciton densities that can be achieved, yet little is known about them. Here, by employing time-resolved photoluminescence measurements, we demonstrate an efficient population of dark excitons via inter-state conversion between X and the spin-indirect intravalley excitons (D) through spin-flip, and between D and the momentum-indirect intervalley excitons (X K ) mediated by the exchange interaction (D+D ←→ X K +X K ). Moreover, we observe a persistent redshift of the X exciton on sub-ns timescales due to strong excitonic screening by the long-lived dense X K exciton. Our results provide a new insight into the many-body interactions between bright and dark excitons, and point to a possibility to employ dark excitons for investigating exciton condensation and valleytronics.
Publisher: Springer Science and Business Media LLC
Date: 23-08-2021
DOI: 10.1038/S41598-021-95700-5
Abstract: Two-dimensional layered materials offer the possibility to create artificial vertically stacked structures possessing an additional degree of freedom— the interlayer twist . We present a comprehensive optical study of artificially stacked bilayers (BLs) MoS $$_2$$ 2 encapsulated in hexagonal BN with interlayer twist angle ranging from 0 $$^{\\circ }$$ ∘ to 60 $$^{\\circ }$$ ∘ using Raman scattering and photoluminescence spectroscopies. It is found that the strength of the interlayer coupling in the studied BLs can be estimated using the energy dependence of indirect emission versus the A $$_\\text {1g}$$ 1g –E $$_\\text {2g}^1$$ 2g 1 energy separation. Due to the hybridization of electronic states in the valence band, the emission line related to the interlayer exciton is apparent in both the natural (2H) and artificial (62 $$^\\circ $$ ∘ ) MoS $$_2$$ 2 BLs, while it is absent in the structures with other twist angles. The interlayer coupling energy is estimated to be of about 50 meV. The effect of temperature on energies and intensities of the direct and indirect emission lines in MoS $$_2$$ 2 BLs is also quantified.
Publisher: American Chemical Society (ACS)
Date: 10-01-2014
DOI: 10.1021/NN500059S
Publisher: American Chemical Society (ACS)
Date: 30-04-2021
Publisher: American Association for the Advancement of Science (AAAS)
Date: 07-06-2019
Abstract: Atomically thin boron nitride is one of the best thermal conductors among semiconductors and insulators.
Publisher: American Chemical Society (ACS)
Date: 14-06-2016
Publisher: American Physical Society (APS)
Date: 26-05-2022
Publisher: Springer Science and Business Media LLC
Date: 13-12-2019
DOI: 10.1038/S41699-019-0133-3
Abstract: In two-dimensional (2D)-semiconductor-based field-effect transistors and optoelectronic devices, metal–semiconductor junctions are one of the crucial factors determining device performance. The Fermi-level (FL) pinning effect, which commonly caused by interfacial gap states, severely limits the tunability of junction characteristics, including barrier height and contact resistance. A tunneling contact scheme has been suggested to address the FL pinning issue in metal–2D-semiconductor junctions, whereas the experimental realization is still elusive. Here, we show that an oxidized-monolayer-enabled tunneling barrier can realize a pronounced FL depinning in indium selenide (InSe) transistors, exhibiting a large pinning factor of 0.5 and a highly modulated Schottky barrier height. The FL depinning can be attributed to the suppression of metal- and disorder-induced gap states as a result of the high-quality tunneling contacts. Structural characterizations indicate uniform and atomically thin-surface oxidation layer inherent from nature of van der Waals materials and atomically sharp oxide–2D-semiconductor interfaces. Moreover, by effectively lowering the Schottky barrier height, we achieve an electron mobility of 2160 cm 2 /Vs and a contact barrier of 65 meV in two-terminal InSe transistors. The realization of strong FL depinning in high-mobility InSe transistors with the oxidized-monolayer presents a viable strategy to exploit layered semiconductors in contact engineering for advanced electronics and optoelectronics.
Publisher: American Chemical Society (ACS)
Date: 09-11-2022
Publisher: IOP Publishing
Date: 11-02-2020
Publisher: American Chemical Society (ACS)
Date: 08-03-2021
Publisher: Springer Science and Business Media LLC
Date: 13-09-2018
DOI: 10.1038/S41467-018-05558-X
Abstract: As hosts for tightly-bound electron-hole pairs carrying quantized angular momentum, atomically-thin semiconductors of transition metal dichalcogenides (TMDCs) provide an appealing platform for optically addressing the valley degree of freedom. In particular, the valleytronic properties of neutral and charged excitons in these systems have been widely investigated. Meanwhile, correlated quantum states involving more particles are still elusive and controversial despite recent efforts. Here, we present experimental evidence for four-particle biexcitons and five-particle exciton-trions in high-quality monolayer tungsten diselenide. Through charge doping, thermal activation, and magnetic-field tuning measurements, we determine that the biexciton and the exciton-trion are bound with respect to the bright exciton and the trion, respectively. Further, both the biexciton and the exciton-trion are intervalley complexes involving dark excitons, giving rise to emissions with large, negative valley polarization in contrast to that of the two-particle excitons. Our studies provide opportunities for building valleytronic quantum devices harnessing high-order TMDC excitations.
Publisher: American Chemical Society (ACS)
Date: 26-10-2023
Publisher: American Chemical Society (ACS)
Date: 21-07-2020
Publisher: IOP Publishing
Date: 22-10-2019
Publisher: Wiley
Date: 21-10-2015
Publisher: American Association for the Advancement of Science (AAAS)
Date: 11-09-2020
Abstract: Photon antibunching, a hallmark of quantum light, is observed from moiré-trapped interlayer excitons in a 2D heterobilayer.
Publisher: Informa UK Limited
Date: 13-04-2023
Publisher: American Chemical Society (ACS)
Date: 22-09-2021
Publisher: Research Square Platform LLC
Date: 28-10-2021
DOI: 10.21203/RS.3.RS-1008445/V1
Abstract: An increase in power consumption necessitates a low-power circuit technology to extend Moore’s law. Low-power transistors, such as tunnel field-effect transistors (TFETs) 1-5 , negative-capacitance field-effect transistors (NC-FETs) 6 , and Dirac-source field-effect transistors (DS-FETs) 7-10 , have been realised to break the thermionic limit of the subthreshold swing (SS). However, a low-power diode rectifier, which breaks the thermionic limit of an ideality factor (η) of 1 at room temperature, has not been proposed yet. In this study, we have realised a DS diode, which exhibits a steep-slope characteristic curve, by utilising the linear density of states (DOSs) of graphene 7 . For the developed DS diode, η 1 for more than two decades of drain current with a minimum value of 0.8, and the rectifying ratio is large ( 10 5 ). The realisation of a DS diode paves the way for the development of low-power electronic circuits.
Publisher: Springer Science and Business Media LLC
Date: 21-01-2014
DOI: 10.1038/SREP03778
Abstract: Rare-earth doped wurtzite-type aluminum nitride ( w -AlN) has great potential for high-efficiency electroluminescent applications over a wide wavelength range. However, because of their large atomic size, it has been difficult to stably dope in idual rare-earth atoms into the w -AlN host lattice. Here we use a reactive flux method under high pressure and high temperature to obtain cerium (Ce) doped w -AlN single crystals with pink-colored luminescence. In order to elucidate the atomic structure of the luminescent centers, we directly observe in idual Ce dopants in w -AlN using annular dark-field scanning transmission electron microscopy. We find that Ce is incorporated as single, isolated atoms inside the w -AlN lattice occupying Al substitutional sites. This new synthesis method represents a new alternative strategy for doping size-mismatched functional atoms into wide band-gap materials.
Publisher: Royal Society of Chemistry (RSC)
Date: 2020
DOI: 10.1039/D0NR04243A
Abstract: The properties of dark excitons and dark trions are investigated in a monolayer of WS 2 encapsulated in hexagonal BN.
Publisher: American Chemical Society (ACS)
Date: 22-04-2020
Publisher: American Chemical Society (ACS)
Date: 23-11-2020
Publisher: American Chemical Society (ACS)
Date: 27-04-2015
DOI: 10.1021/ACS.NANOLETT.5B00098
Abstract: Atomic force microscopy, Kelvin probe force microscopy, and scanning photoluminescence spectroscopy image the progressive postgrowth hydroxylation and hydration of atomically flat Al2O3(0001) under monolayer MoS2, manifested in large work function shifts (100 mV) due to charge transfer (>10(13) cm(-2)) from the substrate and changes in PL intensity, energy, and peak width. In contrast, trapped water between exfoliated graphene and Al2O3(0001) causes surface potential and doping changes one and two orders of magnitude smaller, respectively, and MoS2 grown on hydrophobic hexagonal boron nitride is unaffected by water exposure.
Publisher: American Chemical Society (ACS)
Date: 05-10-2021
Publisher: Springer Science and Business Media LLC
Date: 15-09-2021
DOI: 10.1038/S41467-021-25747-5
Abstract: Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe 2 and WS 2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light.
Publisher: American Physical Society (APS)
Date: 08-07-2022
Publisher: AIP Publishing
Date: 21-09-2015
DOI: 10.1063/1.4931621
Abstract: We report on the photovoltaic effect in a WSe2/MoSe2 heterojunction, demonstrating gate tunable current rectification with on/off ratios of over 104. Spatially resolved photocurrent maps show the photovoltaic effect to originate from the entire overlap region. Compared to WSe2/MoS2 heterostructures, our devices perform better at long wavelengths and yield higher quantum efficiencies, in agreement with Shockley-Queisser theory.
Publisher: Springer Science and Business Media LLC
Date: 20-12-2021
Publisher: Royal Society of Chemistry (RSC)
Date: 2023
DOI: 10.1039/D3NR03168C
Publisher: American Chemical Society (ACS)
Date: 08-06-2023
Publisher: Springer Science and Business Media LLC
Date: 30-05-2022
DOI: 10.1038/S41467-022-30645-5
Abstract: Engineering the properties of quantum materials via strong light-matter coupling is a compelling research direction with a multiplicity of modern applications. Those range from modifying charge transport in organic molecules, steering particle correlation and interactions, and even controlling chemical reactions. Here, we study the modification of the material properties via strong coupling and demonstrate an effective inversion of the excitonic band-ordering in a monolayer of WSe 2 with spin-forbidden, optically dark ground state. In our experiments, we harness the strong light-matter coupling between cavity photon and the high energy, spin-allowed bright exciton, and thus creating two bright polaritonic modes in the optical bandgap with the lower polariton mode pushed below the WSe 2 dark state. We demonstrate that in this regime the commonly observed luminescence quenching stemming from the fast relaxation to the dark ground state is prevented, which results in the brightening of this intrinsically dark material. We probe this effective brightening by temperature-dependent photoluminescence, and we find an excellent agreement with a theoretical model accounting for the inversion of the band ordering and phonon-assisted polariton relaxation.
Publisher: American Physical Society (APS)
Date: 02-03-2023
Publisher: American Chemical Society (ACS)
Date: 03-11-2022
Publisher: Springer Science and Business Media LLC
Date: 04-11-2021
DOI: 10.1038/S41467-021-26715-9
Abstract: The emergence of spatial and temporal coherence of light emitted from solid-state systems is a fundamental phenomenon intrinsically aligned with the control of light-matter coupling. It is canonical for laser oscillation, emerges in the superradiance of collective emitters, and has been investigated in bosonic condensates of thermalized light, as well as exciton-polaritons. Our room temperature experiments show the strong light-matter coupling between microcavity photons and excitons in atomically thin WSe 2 . We evidence the density-dependent expansion of spatial and temporal coherence of the emitted light from the spatially confined system ground-state, which is accompanied by a threshold-like response of the emitted light intensity. Additionally, valley-physics is manifested in the presence of an external magnetic field, which allows us to manipulate K and K’ polaritons via the valley-Zeeman-effect. Our findings validate the potential of atomically thin crystals as versatile components of coherent light-sources, and in valleytronic applications at room temperature.
Publisher: Springer Science and Business Media LLC
Date: 05-07-2018
DOI: 10.1038/S41467-018-05117-4
Abstract: Development of scalable quantum photonic technologies requires on-chip integration of photonic components. Recently, hexagonal boron nitride (hBN) has emerged as a promising platform, following reports of hyperbolic phonon-polaritons and optically stable, ultra-bright quantum emitters. However, exploitation of hBN in scalable, on-chip nanophotonic circuits and cavity quantum electrodynamics (QED) experiments requires robust techniques for the fabrication of high-quality optical resonators. In this letter, we design and engineer suspended photonic crystal cavities from hBN and demonstrate quality ( Q ) factors in excess of 2000. Subsequently, we show deterministic, iterative tuning of in idual cavities by direct-write EBIE without significant degradation of the Q -factor. The demonstration of tunable cavities made from hBN is an unprecedented advance in nanophotonics based on van der Waals materials. Our results and hBN processing methods open up promising avenues for solid-state systems with applications in integrated quantum photonics, polaritonics and cavity QED experiments.
Publisher: Research Square Platform LLC
Date: 23-02-2023
DOI: 10.21203/RS.3.RS-2516689/V1
Abstract: Atomic defects in two-dimensional (2D) semiconductors are promising single exciton traps for achieving quantum emission. However, excessively high density of defects often makes it impractical to optically address the quantum nature of such quasi-particles, preventing strategic exciton engineering. Here, we report a versatile strategy for dilute acceptor impurity doping of monolayer WS 2 , which enables the study of isolated bound excitons. We observe two distinct types of acceptor-bound excitons: one of them is characterized by a large binding energy, allowing its observation at room temperature and the other by an extremely narrow linewidth ( 1 meV), showing strong resemblance to quantum emitters in WSe 2 . Polarization-resolved spectroscopy and magnetic field dependence reveal their strong contribution of free exciton character. In particular, the latter type exhibits characteristics of acceptor-bound dark excitons. Our work demonstrates dilute impurity doping as a promising strategy towards optically addressable quantum states in monolayer semiconductors.
Publisher: Springer Science and Business Media LLC
Date: 26-07-2022
DOI: 10.1038/S41467-022-31849-5
Abstract: An increase in power consumption necessitates a low-power circuit technology to extend Moore’s law. Low-power transistors, such as tunnel field-effect transistors (TFETs), negative-capacitance field-effect transistors (NC-FETs), and Dirac-source field-effect transistors (DS-FETs), have been realised to break the thermionic limit of the subthreshold swing (SS). However, a low-power rectifier, able to overcome the thermionic limit of an ideality factor (η) of 1 at room temperature, has not been proposed yet. In this study, we have realised a DS diode based on graphene/MoS 2 /graphite van der Waals heterostructures, which exhibits a steep-slope characteristic curve, by exploiting the linear density of states (DOSs) of graphene. For the developed DS diode, we obtained η 1 for more than four decades of drain current (η ave_4dec 1) with a minimum value of 0.8, and a rectifying ratio exceeding 10 8 . The realisation of a DS diode represents an additional step towards the development of low-power electronic circuits.
Publisher: American Chemical Society (ACS)
Date: 16-05-2022
Abstract: In artificial van der Waals (vdW) layered devices, twisting the stacking angle has emerged as an effective strategy to regulate the electronic phases and optical properties of these systems. Along with the twist registry, the lattice reconstruction arising from vdW interlayer interaction has also inspired significant research interests. The control of twist angles is significantly important because the moiré periodicity determines the electron propagation length on the lattice and the interlayer electron-electron interactions. However, the moiré periodicity is hard to be modified after the device has been fabricated. In this work, we have demonstrated that the moiré periodicity can be precisely modulated with a localized laser annealing technique. This is achieved with regulating the interlayer lattice mismatch by the mismatched lattice constant, which originates from the variable density of sulfur vacancy generated during laser modification. The existence of sulfur vacancy is further verified by excitonic emission energy and lifetime in photoluminescence measurements. Furthermore, we also discover that the mismatched lattice constant has the equivalent contribution as the twist angle for determining the lattice mismatch. Theoretical modeling elaborates the moiré-wavelength-dependent energy variations at the interface and mimics the evolution of moiré morphology.
Publisher: AIP Publishing
Date: 17-08-2020
DOI: 10.1063/5.0013092
Abstract: Monolayer transition metal dichalcogenide semiconductors, with versatile experimentally accessible exciton species, offer an interesting platform for investigating the interaction between excitons and a Fermi sea of charges. Using hexagonal boron nitride encapsulated monolayer MoSe2, we study the impact of charge density tuning on the A and B series of exciton Rydberg states, including A:1s, A:2s, B:1s, and B:2s. The doping dependence of the A:2s state provides an opportunity to examine such interactions with greatly reduced exciton binding energy and more spatially diffuse structures, and we found that the impact of the Fermi sea becomes much more dramatic compared to the A:1s state. Using photoluminescence upconversion, we verify that the B:2s exciton state displays similar behavior when interacting with the Fermi sea despite being well above the bare bandgap in energy. Photoluminescence and reflection spectra of the A:1s state show clear evidence that the interaction of the exciton with a Fermi sea is best described by the exciton–polaron model, rather than a trion model. Our experimental results demonstrate that overall features of charge interaction are quite generic and highly robust, offering key insights into the dressed many body states in a Fermi sea.
Publisher: American Chemical Society (ACS)
Date: 08-02-2019
Publisher: American Association for the Advancement of Science (AAAS)
Date: 11-2013
Abstract: Electrical contact to graphene is normally done with metal contacts on its flat face, where there are few strong bonding sites for the metal. Wang et al. (p. 614 ) encapsulated graphene with hexagonal boron nitride sheets and made metal contacts along its edge, where bonding orbitals are exposed. The resulting heterostructures had high electronic performance, with room-temperature carrier mobilities near the theoretical phonon-scattering limit.
Publisher: American Chemical Society (ACS)
Date: 11-06-2021
Publisher: Research Square Platform LLC
Date: 25-05-2021
DOI: 10.21203/RS.3.RS-533063/V1
Abstract: The emergence of two-dimensional (2D) magnetic crystals and moiré engineering of van der Waals materials has opened the door for devising new magnetic ground states via competing interactions in moiré superlattices. Although a suite of interesting phenomena, including multi-flavor magnetic states, noncollinear magnetic states, moiré magnon bands and magnon networks, has been predicted in twisted bilayer magnetic crystals, nontrivial magnetic ground states have yet to be realized. Here, by utilizing the stacking-dependent interlayer exchange interactions in CrI3, we demonstrate in small-twist-angle CrI3 bilayers a noncollinear magnetic ground state. It consists of antiferromagnetic (AF) and ferromagnetic (FM) domains and is a result of the competing interlayer AF coupling in the monoclinic stacking regions of the moiré superlattice and the energy cost for forming AF-FM domain walls. Above a critical twist angle of ~ 3°, the noncollinear state transitions to a collinear FM ground state. We further show that the noncollinear magnetic state can be controlled by electrical gating through the doping-dependent interlayer AF interaction. Our results demonstrate the possibility of engineering new magnetic ground states in twisted bilayer magnetic crystals, as well as gate-voltage-controllable high-density magnetic memory storage.
Publisher: Wiley
Date: 15-01-2021
Location: Japan
Start Date: 09-2020
End Date: 11-2024
Amount: $419,778.00
Funder: Australian Research Council
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