Layered transition metal dichalcogenides (TMDs) draw much attention as the key semiconducting material for two-dimensional electrical, optoelectronic, and spintronic devices. For most of these applications, both n- and p-type materials are needed to form junctions and support bipolar carrier conduction. However, typically only one type of doping is stable for a particular TMD. For example, molybdenum disulfide (MoS2) is natively an n-type presumably due to omnipresent electron-donating sulfur vacancies, and stable/controllable p-type doping has not been achieved. The lack of p-type doping hampers the development of charge-splitting p-n junctions of MoS2, as well as limits carrier conduction to spin-degenerate conduction bands instead of the more interesting, spin-polarized valence bands. Traditionally, extrinsic p-type doping in TMDs has been approached with surface adsorption or intercalation of electron-accepting molecules. However, practically stable doping requires substitution of host atoms with dopants where the doping is secured by covalent bonding. In this work, we demonstrate stable p-type conduction in MoS2 by substitutional niobium (Nb) doping, leading to a degenerate hole density of ∼ 3 × 10(19) cm(-3). Structural and X-ray techniques reveal that the Nb atoms are indeed substitutionally incorporated into MoS2 by replacing the Mo cations in the host lattice. van der Waals p-n homojunctions based on vertically stacked MoS2 layers are fabricated, which enable gate-tunable current rectification. A wide range of microelectronic, optoelectronic, and spintronic devices can be envisioned from the demonstrated substitutional bipolar doping of MoS2. From the miscibility of dopants with the host, it is also expected that the synthesis technique demonstrated here can be generally extended to other TMDs for doping against their native unipolar propensity.

Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS2 crystals. The doping readily induces a structural transformation from naturally occurring 2H stacking to 3R stacking. Electronically, a strong interaction of the Nb impurity states with the host valence bands drastically and nonlinearly modifies the electronic band structure with the valence band maximum of multilayer MoS2 at the Γ point pushed upward by hybridization with the Nb states. When thinned down to monolayers, in stark contrast, such significant nonlinear effect vanishes, instead resulting in strong and broadband photoluminescence via the formation of exciton complexes tightly bound to neutral acceptors.

Figure 3. Device characteristics under illumination with λ = 660 nm at V G = 0 V. The incident power ranges from 0 to 130 mW/cm². a) Power dependence of J-V SD characteristics. b) Incident power dependence of the electrical power density P el vs. V SD . c) Logarithmic (linear) power-dependence of V OC (J SC ) extracted from a). This is the post-peer reviewed version of the following article: S.A. Svatek et al. "Gate Tunable Photovoltaic Effect in MoS2 vertical P-N Homostructures" Figure 5. a)-d) Optical images of a quasi-transparent device with MoS 2 :Nb (a, b) and the MoS 2 :Nb -MoS 2 :Fe junction (c, d) in bright field (a, c) and transmission-mode (b, d) on a polycarbonate substrate with Au/Ti (70 nm/15 nm) leads. e) J-V SD curves in dark and under illumination with λ = 660 nm and 80 mW/cm². Fits assuming the Shockley diode model are overlaid as solid lines. f) A photograph of the device. The scale bar refers to the focal plane of the camera.

We prepared undoped and Fe-doped MoS2 layered crystals using a chemical vapor transport method to compare their optical and electrical properties. Optical behaviors of carrier transitions were observed successfully in both undoped and Fe-doped MoS2 samples using reflectance and piezoreflectance. Frequency-dependent photoconductivity (PC) measurements reveal an additional deep Fe doping level for the Fe-doped MoS2 sample. In addition, a longer carrier lifetime was calculated for the Fe-doped MoS2 sample than for the undoped MoS2 sample through PC analysis. Hall measurements were also performed for both samples and indicated that the Fe-doped MoS2 sample exhibited a higher carrier concentration and a lower mobility owing to the effect of Fe dopants. Furthermore, both samples were confirmed to have n-type carriers.

In this research nanoporous structures on p-type GaN:Mg and n-type GaN:Si surfaces were fabricated through a photoelectrochemical (PEC) oxidation and an oxide-removing process. The photoluminescence (PL) intensities of GaN and InGaN∕GaN multi-quantum-well (MQW) structures were enhanced by forming this nanoporous structure to increase light extraction efficiency. The PL emission peaks of an MQW active layer have a blueshift phenomenon from 465.5nm (standard) to 456.0nm (nanoporous) measured at 300K which was caused by partially releasing the compressive strain from the top GaN:Mg layers. The internal quantum efficiency could be increased by a partial strain release that induces a lower piezoelectric field in the active layer. The thermal activation energy of a nanoporous structure (85meV) is higher than the standard one (33meV) from a temperature dependent PL measurement. The internal quantum efficiency and light extraction efficiency of an InGaN∕GaN MQW active layer are significantly enhanced by this nanoporous GaN:Mg surface, and this PEC treated nanoporous structure is suitable for high-power lighting applications.

The construction of the mixed-dimensional van der Waals (vdW) heterostructures with two-dimensional (2D) and one-dimensional (1D) materials can advantageously integrate their respective dimensional properties to produce new device functionalities and/or enhance device performance. In this case, a single semiconductor nanowire (NW) can function as an optical cavity and a gain medium, while the atomically thin 2D material does not strongly absorb the NW's light emission or disturb the optical propagation mode. Therefore, the mixed-dimensional 2D/1D vdW heterostructure might provide a new route to realize high-efficiency light-emitting diodes (LEDs) and/or even electrically driven lasers. Here, we report a LED based on a p-type MoS nanosheet and an n-type CdSe NW. The 2D/1D vdW heterojunction diode exhibits a rectification ratio of ∼24 at V = ±3 V, and a low turn-on voltage of ∼1.5 V. Under the forward bias exceeding the turn-on voltage, the 2D/1D vdW heterojunction exhibits strong electroluminescence centered at ∼709 nm, corresponding to the band-edge emission of the CdSe NW. This novel 2D/1D vdW device, which takes advantages of both 2D and 1D semiconductors, enables potential future applications in electrically driven lasers, high-sensitivity sensors, and transparent flexible devices.

In this work, we used the chemical vapor transport (CVT) method to grow PbI2 crystals using iodine as a self-transporting agent. The crystals’ structure, composition, and uniformity were confirmed by X-ray diffraction (XRD) and electron probe microanalysis (EPMA) measurements. We investigated the band gap energy using absorption spectroscopy measurements. Furthermore, we explored the temperature dependence of the band gap energy, which shifts from 2.346 eV at 300 K to 2.487 eV at 20 K, and extracted the temperature coefficients. A prototype photodetector with a lateral metal–semiconductor–metal (MSM) configuration was fabricated to evaluate its photoelectric properties using a photoconductivity spectrum (PC) and persistent photoconductivity (PPC) experiments. The resonance-like PC peak indicates the excitonic transition in absorption. The photoresponse ILight/IDark-1 is up to 200%.

The next generation of semiconductors for electronics requires materials beyond silicon with increased functionality, performance, and scaling in integrated circuits. [1] Among nanomaterials, 2D semiconducting transition metal dichalcogenides (TMDCs) have becoming promising candidates due to their atomic thickness and nonzero bandgap. [2] As 2D metal-oxide-semiconductor field-effect transistors (MOSFETs) may be scaled down to atomic channel lengths due to the excellent electrostatic integrity and ability to suppress sourcedrain tunneling, [3] great strides and accomplishments have been achieved in TMDC-based electronics, such as the integration of 2D transistors into digital complementary metal-oxide semiconductor (CMOS) and flexible electronics. [4] However, the viability of 2D electronics remains under debate, as the electronic transport in 2D materials exhibits a large hysteresis (or the dependence of the system's state on its history). This is unavoidable and ubiquitous in 2D materials due to the large surface-to-volume ratio (i.e., surface effects are much more pronounced compared to 3D bulk solids). [5] For example, a 2D channel may have more than one possible conductivity at a given gate voltage, depending on how the gate voltage varied through the device history. This history dependence is the basis of 2D memory [6] but clearly undesirable for 2D FETs and related electronic devices that require consistent conductivities at given gate voltages. [7] While MoS 2 n-type metal-oxide semiconductor (NMOS) and MoS 2 -WSe 2 CMOS inverters also have been reported in the literature, [8] there have rarely been reports of hysteresis-free 2D circuitry, a necessary property in electronic reliability. [8c,9] Encapsulation with the van der Waals (vdWs) dielectric, hexagonal boron nitride (hBN), has proven to provide excellent protection of 2D semiconductors from environmental factors. By isolating the semiconductor from the ambient environment, device performance becomes highly stable even at elevated temperatures necessary for future device operation. [10] Using recently developed techniques for mechanically assembling vdWs heterostructures layer-by-layer and electrically edge-contacting graphite electrodes, [11] we fabricated hBN encapsulated high-performance 2D semiconductor transistors, NMOS and Graphene and subsequently discovered layered semiconducting transition metal dichalcogenides (TMDCs) exhibit numerous exotic physical properties and broad potential device applications. These 2D semiconducting TMDCs have become particularly interesting in next-generation electronic device applications due to their atomic thickness and nonzero bandgap. However, as there is no bulk volume, the 2D nature makes the electronic transport in these crystals highly sensitive to the environmental conditions, such as humidity, adsorbates, and trapped charges in neighboring dielectrics. Due to this environmental sensitivity, 2D-based circuits and devices suffer from a large and undesirable environment-induced hysteresis, which must ...