The Bi2O2Se surfaces are well-known to possess 50% Se vacancies, yet they have shown no in-gap states within the indirect bandgap (â¼0.8 eV). We have found that the hidden in-gap states arising from the Se vacancies in a 2 × 1 pattern induce a reduced direct bandgap (â¼0.5 eV). Such a reduced direct bandgap is responsible for the high electron mobility of Bi2O2Se. Moreover, the Bi oxide overlayers of the Bi thin films, formed through air exposure and annealing, unexpectedly exhibit a large direct bandgap (â¼2.1 eV). The simplified fabrication of Bi oxide overlayers provides promise for improving Bi2O2Se electronic devices and enhancing photocatalytic activity.
Recently, it has been revealed that dark excitons play a significant role in optically controlled information processing due to their much longer radiative lifetimes than those of bright ones. For the realizable implementation of the features, it is important to understand and manipulate conditions in which dark excitons could exist. We adopt strain-engineered rippling as a new parameter for the modification of the electronic structure of monolayer MoS2 and demonstrate the efficient conversion of bright to dark excitons via a first-principles study. For rippled monolayer MoS2 above a strain of â¼6.8%, we show that the spin order of the conduction band is reversed and the spin forbidden dark exciton then goes below the bright one.
In energy conversion techniques, two-dimensional (2D) thermoelectric materials with high performance are strongly required. This study scrutinizes the electronic and thermoelectric properties of 2D single-layer (1L) ZrTeSe4 based on first-principles calculations combined with Boltzmann transport theory. First-principles molecular dynamics simulations and phonon calculations confirm the thermodynamic stability of 1L-ZrTeSe4. Furthermore, the electron mobility of 1L-ZrTeSe4 is calculated to be â¼5706 cm2 V-1 s-1, which is much higher than that of the typical 2D semiconducting materials. Intriguingly, the calculated lattice thermal conductivity of 1L-ZrTeSe4 is found to be 3.16 W m-1 K-1 at room temperature, which is relatively smaller than that of 2D transition metal dichalcogenides. The maximum figure of merit ZT of 1L-ZrTeSe4 at 900 K is â¼0.8 for both p- and n-type doping at optimal carrier concentrations. As ZT could be improved through the manipulation of its electronic structure, this is an important clue indicating the enormous potential of 1L-ZrTeSe4 in thermoelectric application.
The oxygen vacancies and hydrogen in oxide semiconductors are regarded as the primary sources of charge carriers and various studies have investigated the effect of hydrogen on the properties of oxide semiconductors. However, the carrier generation mechanism between hydrogen and oxygen vacancies in an a-IGZO semiconductor has not yet been clearly examined. In this study we investigated the effect of hydrogen and the variation mechanisms of electrical properties of a thin film supplied with hydrogen from the passivation layer. SiOx and SiNx, which are used as passivation or gate insulator layers in the semiconductor process, respectively, were placed on the top or bottom of an a-IGZO semiconductor to determine the amount of hydrogen penetrating the a-IGZO active layer. The hydrogen diffusion depth was sufficiently deep to affect the entire thin semiconductor layer. A large amount of hydrogen in SiNx directly affects the electrical resistivity of a-IGZO semiconductor, whereas in SiOx, it induces a different behavior from that in SiNx, such as inducing an oxygen reaction and O-H bond behavior change at the interface of an a-IGZO semiconductor. Moreover, the change in electrical resistivity owing to the contribution of free electrons could be varied based on the bonding method of hydrogen and oxygen.
While 2D transition metal dichalcogenides (TMDs) are promising building blocks for various optoelectronic applications, limitations remain for multilayered TMD-based photodetectors: an indirect bandgap and a short carrier lifetime by strongly bound excitons. Accordingly, multilayered TMDs with a direct bandgap and an enhanced carrier lifetime are required for the development of various optoelectronic devices. Here, periodically arrayed nanopore structures (PANS) are proposed for improving the efficiency of multilayered p-WSe2 /n-MoS2 phototransistors. Density functional theory calculations as well as photoluminescence and time-resolved photoluminescence measurements are performed to characterize the photodetector figures of merit of multilayered p-WSe2 /n-MoS2 heterostructures with PANS. The characteristics of the heterojunction devices with PANS reveal an enhanced responsivity and detectivity measured under 405 nm laser excitation, which at 1.7 × 104 A W-1 and 1.7 × 1013 Jones are almost two orders of magnitude higher than those of pristine devices, 3.6 × 102 A W-1 and 3.6 × 1011 Jones, respectively. Such enhanced optical properties of WSe2 /MoS2 heterojunctions with PANS represent a significant step toward next-generation optoelectronic applications.
Beyond the general purpose of noble gas ion sputtering, which is to achieve functional defect engineering of two-dimensional (2D) materials, we herein report another positive effect of low-energy (100 eV) He+ ion irradiation: converting n-type MoS2 to p-type by electron capture through the migration of the topmost S atoms. The electron capture ability via He+ ion irradiation is valid for supported bilayer MoS2; however, it is limited at supported monolayer MoS2 because the charges on the underlying substrates transfer into the monolayer under the current condition for He+ ion irradiation. Our technique provides a stable and universal method for converting n-type 2D transition metal dichalcogenides (TMDs) into p-type semiconductors in a controlled fashion using low-energy He+ ion irradiation.
As novel applications of oxide semiconductors are realized, various structural devices and integrated circuits are being proposed, and the gate-overlay defect phenomenon is becoming more diverse in its effects. Herein, the electrical properties of the transistor that depend on the geometry between the gate and the semiconductor layer are analyzed, and the specific phenomena associated with the degree of overlap are reproduced. In the semiconductor layer, where the gate electrode is not overlapped, it is experimentally shown that a dual current is generated, and the results of 3D simulations confirm that the magnitude of the current increases as the parasitic current moves away from the gate electrode. The generation and path of the parasitic current are then represented visually through laser-enhanced 2D transport measurements; consequently, the flow of the dual current in the transistor is verified to be induced by the electrical potential imbalance in the semiconductor active layer, where the gate electrodes do not overlap.
Quantum confinements, especially quantum in narrow wells, have been investigated because of their controllability over electrical parameters. For example, quantum dots can emit a variety of photon wavelengths even for the same material depending on their particle size. More recently, the research into two-dimensional (2D) materials has shown the availability of several quantum mechanical phenomenon confined within a sheet of materials. Starting with the gapless semimetal properties of graphene, current research has begun into the excitons and their properties within 2D materials. Even for simple 2D systems, experimental results often offer surprising results, unexpected from traditional studies. We investigated a coupled quantum well system using 2D hexagonal boron nitride (hBN) barrier as well as 2D tungsten disulfide (WS2) semiconductor arranged in stacked structures to study the various 2D to 2D interactions. We determined that for hexagonal boron nitride-tungsten disulfide (hBN/WS2) quantum well stacks, the interaction between successive wells resulted in decreasing bandgap, and the effect was pronounced even over a large distance of up to four stacks. Additionally, we observed that a single layer of isolating hBN barriers significantly reduces interlayer interaction between WS2 layers, while still preserving the interwell interactions in the alternative hBN/WS2 structure. The methods we used for the study of coupled quantum wells here show a method for determining the respective exciton energy levels and trion energy levels within 2D materials and 2D materials-based structures. Renormalization energy levels are the key in understanding conductive and photonic properties of stacked 2D materials.
Silicon has been widely used in the microelectronics industry. However, its photonic applications are restricted to visible and partial near-infrared spectral range owing to its fundamental optical bandgap (1.12 eV). With recent advances in strain engineering, material properties, including optical bandgap, can be tailored considerably. This paper reports the strain-induced shrinkage in the Si bandgap, providing photosensing well beyond its fundamental absorption limit in Si nanomembrane (NM) photodetectors (PDs). The Si-NM PD pixels were mechanically stretched (biaxially) by a maximum strain of ~3.5% through pneumatic pressure-induced bulging, enhancing photoresponsivity and extending the Si absorption limit up to 1550 nm, which is the essential wavelength range of the lidar sensors for obstacle detection in self-driving vehicles. The development of deformable three-dimensional optoelectronics via gas pressure-induced bulging also facilitated the realization of unique device designs with concave and convex hemispherical architectures, which mimics the electronic prototypes of biological eyes.
Engineering phase transitions or finding new polymorphs offers tremendous opportunities for developing functional materials. We reveal that the thermally driven desulfurization of single-crystalline MoS2 samples improves transport properties by reducing the band gap and further induces metallization. Semi-desulfurization, i.e., removal of the topmost S layer, results in the placement of the exposed Mo layers directly on top of the following sub-layers, together with the bottom S layer of the top layer. This homonuclear (AA) stacking derived from the AA' stacking of the hexagonal (2H) phase is retained even after further desulfurization of the remaining bottom S layer, i.e., full desulfurization of the top layer. Our findings fundamentally explain why the 2H phase of TMDs is characterized by AA' stacking.
Understanding the collaborative behaviors of the excitons and phonons that result from light-matter interactions is important for interpreting and optimizing the underlying fundamental physics at work in devices made from atomically thin materials. In this study, the generation of exciton-coupled phonon vibration from molybdenum disulfide (MoS2 ) nanosheets in a pre-excitonic resonance condition is reported. A strong rise-to-decay profile for the transient second-harmonic generation (TSHG) of the probe pulse is achieved by applying substantial (20%) beam polarization normal to the nanosheet plane, and tuning the wavelength of the pump beam to the absorption of the A-exciton. The time-dependent TSHG signals clearly exhibit acoustic phonon generation at vibration modes below 10 cm-1 (close to the Γ point) after the photoinduced energy is transferred from exciton to phonon in a nonradiative fashion. Interestingly, by observing the TSHG signal oscillation period from MoS2 samples of varying thicknesses, the speed of the supersonic waves generated in the out-of-plane direction (Mach 8.6) is generated. Additionally, TSHG microscopy reveals critical information about the phase and amplitude of the acoustic phonons from different edge chiralities (armchair and zigzag) of the MoS2 monolayers. This suggests that the technique could be used more broadly to study ultrafast physics and chemistry in low-dimensional materials and their hybrids with ultrahigh fidelity.
Searching for novel two-dimensional (2D) semiconducting materials is a challenging issue. We investigate novel 2D semiconductors ZrNCl and HfNCl which would be isolated to single layers from van der Waals layered bulk materials, i.e., ternary transition-metal nitride halides. Their isolations are unquestionably supported through an investigation of their cleavage energies as well as their thermodynamic stability based on the ab initio molecular dynamics and phonon dispersion calculations. Strain engineering is found to be available for both single-layer (1L) ZrNCl and 1L-HfNCl, where a transition from an indirect to direct band gap is attained under a tensile strain. It is also found that 1L-ZrNCl has an excellent electron mobility of about 1.2 × 103 cm2 V-1 s-1, which is significantly higher than that of 1L-MoS2. Lastly, it is indicated that these systems have good thermoelectric properties, i.e., high Seebeck coefficient and high power factor. With these findings, 1L-ZrNCl and 1L-HfNCl would be novel promising 2D materials for a wide range of optoelectronic and thermoelectric applications.
We report on the thickness-dependent Raman spectroscopy of ultrathin silicon (Si) nanomembranes (NMs), whose thicknesses range from 2 to 18 nm, using several excitation energies. We observe that the Raman intensity depends on the thickness and the excitation energy due to the combined effects of interference and resonance from the band-structure modulation. Furthermore, confined acoustic phonon modes in the ultrathin Si NMs were observed in ultralow-frequency Raman spectra, and strong thickness dependence was observed near the quantum limit, which was explained by calculations based on a photoelastic model. Our results provide a reliable method with which to accurately determine the thickness of Si NMs with thicknesses of less than a few nanometers.
The activation of open coordination sites (OCSs) in metal-organic frameworks (MOFs), i.e., the removal of solvent molecules coordinated at the OCSs, is an essential step that is required prior to the use of MOFs in potential applications such as gas chemisorption, separation, and catalysis because OCSs often serve as key sites in these applications. Recently, we developed a "chemical activation" method involving dichloromethane (DCM) treatment at room temperature, which is considered to be a promising alternative to conventional thermal activation (TA), because it does not require the application of external thermal energy, thereby preserving the structural integrity of the MOFs. However, strongly coordinating solvents such as N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), and dimethyl sulfoxide (DMSO) are difficult to remove solely with the DCM treatment. In this report, we demonstrate a multiple coordination exchange (CE) process executed initially with acetonitrile (MeCN), methanol (MeOH), or ethanol (EtOH) and subsequently with DCM to achieve the complete activation of OCSs that possess strong extracoordination. Thus, this process can serve as an effective "chemical route" to activation at room temperature that does not require applying heat. To the best of our knowledge, no previous study has demonstrated the activation of OCSs using this multiple CE process, although MeOH and/or DCM has been popularly used in pretreatment steps prior to the TA process. Using MOF-74(Ni), we demonstrate that this multiple CE process can safely activate a thermally unstable MOF without inflicting structural damage. Furthermore, on the basis of in situ 1H nuclear magnetic resonance (1H NMR) and Raman studies, we propose a plausible mechanism for the activation behavior of multiple CE.
Schottky barrier height and carrier polarity are seminal concepts for a practical device application of the interface between semiconductor and metal electrode. Investigation of those concepts is usually made by a conventional method such as the Schottky-Mott rule, incorporating the metal work function and semiconductor electron affinity, or the Fermi level pinning effect, resulting from the metal-induced gap states. Both manners are, however, basically applied to the bulk semiconductor metal contacts. To explore few-layer black phosphorus metal contacts far from the realm of bulk, we propose a new method to determine the Schottky barrier by scrutinizing the layer-by-layer phosphorus electronic structure from the first-principles calculation combined with the state-of-the-art band unfolding technique. In this study, using the new method, we calculate the Schottky barrier height and determine the contact polarity of Ti, Sc, and Al metal contacts to few-layer (mono-, bi-, tri-, and quadlayer) black phosphorus. This gives a significant physical insight toward the utmost layer-by-layer manipulation of electronic properties of few-layer semiconductor metal contacts.
For the practical device application of the two-dimensional semiconducting MoS2, it is a critical issue to manipulate the electronic and magnetic properties locally at its contact to the metal electrode. For the tuning of those properties, we have proposed the vacancy-defective 1L-MoS2 or the hydrogenated 1L-MoS2 at the metal [Co(0001) or Ni(111)] contacts and performed first-principles electronic structure calculations. By controlling the atomic vacancy defects and the hydrogen coverages, we investigate the Schottky barrier heights and charge and spin transfers at the interface. Our findings provide a physical insight into the practical device design using the two-dimensional MoS2.
Functionalized X-Bi bilayers (X = Ga, In, and Tl) with halogens bonded on their both sides have been recently claimed to be the giant topological insulators due to the strong band inversion strengths. Employing the first-principles electronic structure calculation, we find the topological band order transition from the order p - p - s of the X-Bi bilayers with halogens on their both sides to the new order p - s - p of the bilayers (especially for X = Ga and In) with halogen on one side and hydrogen on the other side, where the asymmetric hydrogen bonding simulates the substrate. We further find that the p - s bulk band gap of the bilayer bearing the new order p - s - p sensitively depends on the electric field, which enables a meaningful engineering of the quantum spin Hall edge state by controlling the external electric field.
The atomic-layered semiconducting materials of transition metal dichalcogenides are considered effective light sources with both potential applications in thin and flexible optoelectronics and novel functionalities. In spite of the great interest in optoelectronic properties of two-dimensional transition metal dichalcogenides, the excitonic properties still need to be addressed, specifically in terms of the interlayer interactions. Here, we report the distinct behavior of the A and B excitons in the presence of interlayer interactions of layered MoS2 crystals. Micro-photoluminescence spectroscopic studies reveal that on the interlayer interactions in double layer MoS2 crystals, the emission quantum yield of the A exciton is drastically changed, whereas that of the B exciton remains nearly constant for both single and double layer MoS2 crystals. First-principles density functional theory calculations confirm that a significant charge redistribution occurs in the double layer MoS2 due to the interlayer interactions producing a local electric field at the interfacial region. Analogous to the quantum-confined Stark effect, we suggest that the distinct behavior of the A and B excitons can be explained by a simplified band-bending model.
Investigating a theoretical model of the optical-field-induced current in dielectrics driven by strong few-cycle laser pulses, we propose an asymmetric conducting of the current by forming a heterojunction made of two distinct dielectrics with a low hole mass (m_{h}^{*}âªm_{e}^{*}) and low electron mass (m_{e}^{*}âªm_{h}^{*}), respectively. This proposition introduces the novel concept of a petahertz (10^{15} Hz) diode to rectify the current in the petahertz domain, which should be a key ingredient for the electric signal manipulation of future light-wave electronics. Further, we suggest the candidate dielectrics for the heterojunction.
Open coordination sites (OCSs) in metal-organic frameworks (MOFs) often function as key factors in the potential applications of MOFs, such as gas separation, gas sorption, and catalysis. For these applications, the activation process to remove the solvent molecules coordinated at the OCSs is an essential step that must be performed prior to use of the MOFs. To date, the thermal method performed by applying heat and vacuum has been the only method for such activation. In this report, we demonstrate that methylene chloride (MC) itself can perform the activation role: this process can serve as an alternative "chemical route" for the activation that does not require applying heat. To the best of our knowledge, no previous study has demonstrated this function of MC, although MC has been popularly used in the pretreatment step prior to the thermal activation process. On the basis of a Raman study, we propose a plausible mechanism for the chemical activation, in which the function of MC is possibly due to its coordination with the Cu(2+) center and subsequent spontaneous decoordination. Using HKUST-1 film, we further demonstrate that this chemical activation route is highly suitable for activating large-area MOF films.
