Tunable spin injection and detection across a van der Waals interface.

Van der Waals heterostructures with two-dimensional magnets offer a magnetic junction with an atomically sharp and clean interface. This attribute ensures that the magnetic layers maintain their intrinsic spin-polarized electronic states and spin-flipping scattering processes at a minimum level, a trait that can expand spintronic device functionalities. Here, using a van der Waals assembly of ferromagnetic Fe3GeTe2 with non-magnetic hexagonal boron nitride and WSe2 layers, we demonstrate electrically tunable, highly transparent spin injection and detection across the van der Waals interfaces. By varying an electrical bias, the net spin polarization of the injected carriers can be modulated and reversed in polarity, which leads to sign changes of the tunnelling magnetoresistance. We attribute the spin polarization reversals to sizable contributions from high-energy localized spin states in the metallic ferromagnet, so far inaccessible in conventional magnetic junctions. Such tunability of the spin-valve operations opens a promising route for the electronic control of next-generation low-dimensional spintronic device applications.

Van der Waals heterojunction diode composed of WS2 flake placed on p-type Si substrate.

P-N junctions represent the fundamental building blocks of most semiconductors for optoelectronic functions. This work demonstrates a technique for forming a WS2/Si van der Waals junction based on mechanical exfoliation. Multilayered WS2 nanoflakes were exfoliated on the surface of bulk p-type Si substrates using a polydimethylsiloxane stamp. We found that the fabricated WS2/Si p-n junctions exhibited rectifying characteristics. We studied the effect of annealing processes on the performance of the WS2/Si van der Waals p-n junction and demonstrated that annealing improved its electrical characteristics. However, devices with vacuum annealing have an enhanced forward-bias current compared to those annealed in a gaseous environment. We also studied the top-gate-tunable rectification characteristics across the p-n junction interface in experiments as well as density functional theory calculations. Under various temperatures, Zener breakdown occurred at low reverse-bias voltages, and its breakdown voltage exhibited a negative coefficient of temperature. Another breakdown voltage was observed, which increased with temperature, suggesting a positive coefficient of temperature. Therefore, such a breakdown can be assigned to avalanche breakdown. This work demonstrates a promising application of two-dimensional materials placed directly on conventional bulk Si substrates.

Electrical and photo-electrical properties of MoS2 nanosheets with and without an Al2O3 capping layer under various environmental conditions.

The electrical and photo-electrical properties of exfoliated MoS2 were investigated in the dark and in the presence of deep ultraviolet (DUV) light under various environmental conditions (vacuum, N2 gas, air, and O2 gas). We examined the effects of environmental gases on MoS2 flakes in the dark and after DUV illumination through Raman spectroscopy and found that DUV light induced red and blue shifts of peaks (E12 g and A1 g) position in the presence of N2 and O2 gases, respectively. In the dark, the threshold voltage in the transfer characteristics of few-layer (FL) MoS2 field-effect transistors (FETs) remained almost the same in vacuum and N2 gas but shifted toward positive gate voltages in air or O2 gas because of the adsorption of oxygen atoms/molecules on the MoS2 surface. We analyzed light detection parameters such as responsivity, detectivity, external quantum efficiency, linear dynamic range, and relaxation time to characterize the photoresponse behavior of FL-MoS2 FETs under various environmental conditions. All parameters were improved in their performances in N2 gas, but deteriorated in O2 gas environment. The photocurrent decayed with a large time constant in N2 gas, but decayed with a small time constant in O2 gas. We also investigated the characteristics of the devices after passivating by Al2O3 film on the MoS2 surface. The devices became almost hysteresis-free in the transfer characteristics and stable with improved mobility. Given its outstanding performance under DUV light, the passivated device may be potentially used for applications in MoS2-based integrated optoelectronic circuits, light sensing devices, and solar cells.

Ultraviolet-light-driven doping modulation in chemical vapor deposition grown graphene.

The tuning of charge carrier density of graphene is an essential factor to achieve the integration of high-efficiency electronic and optoelectronic devices. We demonstrate the reversible doping in graphene using deep ultraviolet (UV) irradiation and treatment with O2 and N2 gases. The Dirac point shift towards a positive gate voltage of chemical vapor deposition grown graphene field-effect transistors confirms the p-type doping, which is observed under UV irradiation and treatment with O2 gas, while it restores its pristine state after treatment with N2 gas under UV irradiation. The emergence of an additional peak in the X-ray photoelectron spectra during UV irradiation and treatment with O2 gas represents the oxidation of graphene, and the elimination of this peak during UV irradiation and treatment with N2 gas reveals the restoration of graphene in its pristine state. The shift in the G and 2D bands in Raman spectra towards higher and then lower wavenumber also suggests p-type doping and then reversible doping in graphene. The controlled doping and its reversibility in large area grown graphene offer a new vision for electronic applications.

Chemical doping of MoS2 multilayer by p-toluene sulfonic acid.

We report the tailoring of the electrical properties of mechanically exfoliated multilayer (ML) molybdenum disulfide (MoS2) by chemical doping. Electrical charge transport and Raman spectroscopy measurements revealed that the p-toluene sulfonic acid (PTSA) imposes n-doping in ML MoS2. The shift of threshold voltage for ML MoS2 transistor was analyzed as a function of reaction time. The threshold voltage shifted toward more negative gate voltages with increasing reaction time, which indicates an n-type doping effect. The shift of the Raman peak positions was also analyzed as a function of reaction time. PTSA treatment improved the field-effect mobility by a factor of ~4 without degrading the electrical characteristics of MoS2 devices.

Improving the electrical properties of graphene layers by chemical doping.

Although the electronic properties of graphene layers can be modulated by various doping techniques, most of doping methods cost degradation of structural uniqueness or electrical mobility. It is matter of huge concern to develop a technique to improve the electrical properties of graphene while sustaining its superior properties. Here, we report the modification of electrical properties of single- bi- and trilayer graphene by chemical reaction with potassium nitrate (KNO3) solution. Raman spectroscopy and electrical transport measurements showed the n-doping effect of graphene by KNO3. The effect was most dominant in single layer graphene, and the mobility of single layer graphene was improved by the factor of more than 3. The chemical doping by using KNO3 provides a facile approach to improve the electrical properties of graphene layers sustaining their unique characteristics.

Modification of the structural and electrical properties of graphene layers by Pt adsorbates.

The properties of graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of platinum (Pt) metal on exfoliated single, bi- and trilayer graphene and on chemical vapor deposition-grown single-layer graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that Pt affects the structure as well as the electronic properties of graphene. The shift of peak frequencies, intensities and widths of the Raman bands were analyzed after the deposition of Pt with different thicknesses (1, 3, 5 nm) on the graphene. The shifts in the G and 2D peak positions of the Raman spectra indicate the n-type doping effect by the Pt metal. The doping effect was also confirmed by gate-voltage dependent resistivity measurements. The doping effect by the Pt metal is stable under ambient conditions, and the doping intensity increases with the increasing Pt deposition without inducing a severe degradation of the charge carrier mobility.

Broadening spectral responses and achieving environmental stability in SnS2/Ag-NPs/HfO2 flexible phototransistors.

Layered two-dimensional (2D) materials have gained popularity thanks to their atomically thin physique and strong coupling with light. Here, we investigated a wide band gap (≥ 2 eV) 2D material, i.e., tin disulfide (SnS2), and decorated it with silver nanoparticles, Ag-NPs, for broadband photodetection. Our results show that the SnS2/Ag-NPs devices exhibit broadband photodetection ranging from the ultraviolet to near-infrared (250-1050 nm) spectrum with decreased rise/decay times from 8/20 s to 7/16 s under 250 nm wavelength light compared to the bare SnS2 device. This is attributed to the localized surface plasmon resonance effect and the wide band gap of SnS2 crystal. Furthermore, the HfO2-passivated SnS2/Ag-NPs devices exhibited high photodetection performance in terms of photoresponsivity (∼12 500 A W-1), and external quantum efficiency (∼6 × 106%), which are significantly higher compared to those of bare SnS2. Importantly, after HfO2 passivation, the SnS2/Ag-NPs photodetector maintained the stable performance for several weeks with merely ∼5.7% reduction in photoresponsivity. Lastly, we fabricated a flexible SnS2/Ag-NPs photodetector, which shows excellent and stable performance under various bending curvatures (0, 20, and 10 mm), as it retains ∼80% of its photoresponsivity up to 500 bending cycles. Thus, our study provides a simple route to realize broadband and stable photoactivity in flexible 2D material-based devices.

Optically Reconfigurable Complementary Logic Gates Enabled by Bipolar Photoresponse in Gallium Selenide Memtransistor.

To avoid the complexity of the circuit for in-memory computing, simultaneous execution of multiple logic gates (OR, AND, NOR, and NAND) and memory behavior are demonstrated in a single device of oxygen plasma-treated gallium selenide (GaSe) memtransistor. Resistive switching behavior with RON /ROFF ratio in the range of 104 to 106 is obtained depending on the channel length (150 to 1600 nm). Oxygen plasma treatment on GaSe film created shallow and deep-level defect states, which exhibit carriers trapping/de-trapping, that lead to negative and positive photoconductance at positive and negative gate voltages, respectively. This distinguishing feature of gate-dependent transition of negative to positive photoconductance encourages the execution of four logic gates in the single memory device, which is elusive in conventional memtransistor. Additionally, it is feasible to reversibly switch between two logic gates by just adjusting the gate voltages, e.g., NAND/NOR and AND/NAND. All logic gates presented high stability. Additionally, memtransistor array (1×8) is fabricated and programmed into binary bits representing ASCII (American Standard Code for Information Interchange) code for the uppercase letter "N". This facile device configuration can provide the functionality of both logic and memory devices for emerging neuromorphic computing.

A Novel Biosensing Approach: Improving SnS2 FET Sensitivity with a Tailored Supporter Molecule and Custom Substrate.

The exclusive features of two-dimensional (2D) semiconductors, such as high surface-to-volume ratios, tunable electronic properties, and biocompatibility, provide promising opportunities for developing highly sensitive biosensors. However, developing practical biosensors that can promptly detect low concentrations of target analytes remains a challenging task. Here, a field-effect-transistor comprising n-type transition metal dichalcogenide tin disulfide (SnS2 ) is developed over the hexagonal boron nitride (h-BN) for the detection of streptavidin protein (Strep.) as a target analyte. A self-designed receptor based on the pyrene-lysine conjugated with biotin (PLCB) is utilized to maintain the sensitivity of the SnS2 /h-BN FET because of the π-π stacking. The detection capabilities of SnS2 /h-BN FET are investigated using both Raman spectroscopy and electrical characterizations. The real-time electrical measurements exhibit that the SnS2 /h-BN FET is capable of detecting streptavidin at a remarkably low concentration of 0.5 pm, within 13.2 s. Additionally, the selectivity of the device is investigated by measuring its response against a Cow-like serum egg white protein (BSA), having a comparative molecular weight to that of the streptavidin. These results indicate a high sensitivity and rapid response of SnS2 /h-BN biosensor against the selective proteins, which can have significant implications in several fields including point-of-care diagnostics, drug discovery, and environmental monitoring.

Reconfigurable carrier type and photodetection of MoTe2 of various thicknesses by deep ultraviolet light illumination.

Tuning of the Fermi level in transition metal dichalcogenides (TMDCs) leads to devices with excellent electrical and optical properties. In this study, we controlled the Fermi level of MoTe2 by deep ultraviolet (DUV) light illumination in different gaseous environments. Specifically, we investigated the reconfigurable carrier type of an intrinsic p-MoTe2 flake that gradually transformed into n-MoTe2 after illumination with DUV light for 30, 60, 90, 120, 160, 250, 500, 900, and 1200 s in a nitrogen (N2) gas environment. Subsequently, we illuminated this n-MoTe2 sample with DUV light in oxygen (O2) gas and reversed its carrier polarity toward p-MoTe2. However, using this doping scheme to reveal the effect of DUV light on various layers (3-30 nm) of MoTe2 is challenging. The DUV + N2 treatment significantly altered the polarity of MoTe2 of different thicknesses from p-type to n-type under the DUV + N2 treatment, but the DUV + O2 treatment did not completely alter the polarity of thicker n-MoTe2 flakes to p-type. In addition, we investigated the photoresponse of MoTe2 after DUV light treatment in N2 and O2 gas environments. From the time-resolved photoresponsivity at different polarity states of MoTe2, we have shown that the response time of the DUV + O2 treated p-MoTe2 is faster than that of the pristine and doped n-MoTe2 films. These carrier polarity modulations and photoresponse paves the way for wider applications of MoTe2 in optoelectronic devices.

Low-Power Negative-Differential-Resistance Device for Sensing the Selective Protein via Supporter Molecule Engineering.

Van der Waals (vdW) heterostructures composed of atomically thin two-dimensional (2D) materials have more potential than conventional metal-oxide semiconductors because of their tunable bandgaps, and sensitivities. The remarkable features of these amazing vdW heterostructures are leading to multi-functional logic devices, atomically thin photodetectors, and negative differential resistance (NDR) Esaki diodes. Here, an atomically thin vdW stacking composed of p-type black arsenic (b-As) and n-type tin disulfide (n-SnS2 ) to build a type-III (broken gap) heterojunction is introduced, leading to a negative differential resistance device. Charge transport through the NDR device is investigated under electrostatic gating to achieve a high peak-to-valley current ratio (PVCR), which improved from 2.8 to 4.6 when the temperature is lowered from 300 to 100 K. At various applied-biasing voltages, all conceivable tunneling mechanisms that regulate charge transport are elucidated. Furthermore, the real-time response of the NDR device is investigated at various streptavidin concentrations down to 1 pm, operating at a low biasing voltage. Such applications of NDR devices may lead to the development of cutting-edge electrical devices operating at low power that may be employed as biosensors to detect a variety of target DNA (e.g., ct-DNA) and protein (e.g., the spike protein associated with COVID-19).

The non-volatile electrostatic doping effect in MoTe2 field-effect transistors controlled by hexagonal boron nitride and a metal gate.

The electrical and optical properties of transition metal dichalcogenides (TMDs) can be effectively modulated by tuning their Fermi levels. To develop a carrier-selectable optoelectronic device, we investigated intrinsically p-type MoTe2, which can be changed to n-type by charging a hexagonal boron nitride (h-BN) substrate through the application of a writing voltage using a metal gate under deep ultraviolet light. The n-type part of MoTe2 can be obtained locally using the metal gate pattern, whereas the other parts remain p-type. Furthermore, we can control the transition rate to n-type by applying a different writing voltage (i.e., - 2 to - 10 V), where the n-type characteristics become saturated beyond a certain writing voltage. Thus, MoTe2 was electrostatically doped by a charged h-BN substrate, and it was found that a thicker h-BN substrate was more efficiently photocharged than a thinner one. We also fabricated a p-n diode using a 0.8 nm-thick MoTe2 flake on a 167 nm-thick h-BN substrate, which showed a high rectification ratio of ~ 10-4. Our observations pave the way for expanding the application of TMD-based FETs to diode rectification devices, along with optoelectronic applications.

Ultrafast and Highly Stable Photodetectors Based on p-GeSe/n-ReSe2 Heterostructures.

Two-dimensional transition-metal dichalcogenide (2D-TMD) semiconductors and their van der Waals heterostructures (vdWHs) have attracted great attention because of their tailorable band-engineering properties and provide a propitious platform for next-generation extraordinary performance energy-harvesting devices. Herein, we reported unique and unreported germanium selenide/rhenium diselenide (p-GeSe/n-ReSe2) 2D-TMD vdWH photodetectors for extremely sensitive and high-performance photodetection in the broadband spectral range (visible and near-infrared range). A high and gate-tunable rectification ratio (RR) of 7.34 × 105 is achieved, stemming from the low Schottky barrier contacts and sharp interfaces of the p-GeSe/n-ReSe2 2D-TMD vdWHs. In addition, a noticeably high responsivity (R = 2.89 × 105 A/W) and specific detectivity (D* = 4.91 × 1013 Jones), with good external quantum efficiency (EQE = 6.1 × 105) are obtained because of intralayer and interlayer transition of excitations, enabling the broadband photoresponse (λ = 532-1550 nm) at room temperature. Furthermore, fast response times of 16-20 µs are estimated under the irradiated laser of λ = 1550 nm because of interlayer exciton transition. Such a TMD-based compact system offers an opportunity for the realization of high-performance broadband infrared photodetectors.

Deep-Ultraviolet (DUV)-Induced Doping in Single Channel Graphene for Pn-Junction.

The electronic properties of single-layer, CVD-grown graphene were modulated by deep ultraviolet (DUV) light irradiation in different radiation environments. The graphene field-effect transistors (GFETs), exposed to DUV in air and pure O2, exhibited p-type doping behavior, whereas those exposed in vacuum and pure N2 gas showed n-type doping. The degree of doping increased with DUV exposure time. However, n-type doping by DUV in vacuum reached saturation after 60 min of DUV irradiation. The p-type doping by DUV in air was observed to be quite stable over a long period in a laboratory environment and at higher temperatures, with little change in charge carrier mobility. The p-doping in pure O2 showed ~15% de-doping over 4 months. The n-type doping in pure N2 exhibited a high doping effect but was highly unstable over time in a laboratory environment, with very marked de-doping towards a pristine condition. A lateral pn-junction of graphene was successfully implemented by controlling the radiation environment of the DUV. First, graphene was doped to n-type by DUV in vacuum. Then the n-type graphene was converted to p-type by exposure again to DUV in air. The n-type region of the pn-junction was protected from DUV by a thick double-coated PMMA layer. The photocurrent response as a function of Vg was investigated to study possible applications in optoelectronics.

WSe2 Homojunction p-n Diode Formed by Photoinduced Activation of Mid-Gap Defect States in Boron Nitride.

A single nanoflake lateral p-n diode (in-plane) based on a two-dimensional material can facilitate electronic architecture miniaturization. Here, a novel lateral homojunction p-n diode of a single WSe2 nanoflake is fabricated by photoinduced doping via optical excitation of defect states in an h-BN nanoflake upon illumination. This lateral diode is fabricated using a mechanical exfoliation technique by stacking the WSe2 nanoflake partially on the h-BN and Si substrates. The carrier type in the part of the WSe2 film on the h-BN substrate is inverted and a built-in potential difference is formed, ranging from 5.0 to 4.50 eV, which is measured by Kelvin probe force microscopy. The contact potential difference across the junction of p-WSe2 and n-WSe2 is found to be ∼492 mV. The lateral diode shows an excellent rectification ratio, up to ∼3.9 × 104, with an ideality factor of ∼1.1. A typical self-biased photovoltaic behavior is observed at the p-n junction upon the illumination of incident light, that is, a positive open-circuit voltage (Voc) is generated, that is, voltage obtained (at Ids = 0 V), and also a negative short-circuit current (Isc) is generated, that is, current obtained (at Vds = 0 V). The presence of built-in potential in the proposed homojunction diode establishes Isc and Voc upon illumination, which can be implemented for a self-powered photovoltaic system in future electronics. The proposed doping technique can be effectively applied to form planar homojunction devices without a photoresist for future electronic and optoelectronic applications.

WS2/GeSe/WS2 Bipolar Transistor-Based Chemical Sensor with Fast Response and Recovery Times.

Vertical heterostructures of transition-metal dichalcogenide semiconductors have attracted considerable attention and offer new opportunities in electronics and optoelectronics for the development of innovative and multifunctional devices. Here, we designed a novel and compact vertically stacked two-dimensional (2D) n-WS2/p-GeSe/n-WS2 van der Waals (vdW) heterojunction bipolar transistor (2D-HBT)-based chemical sensor. The performance of the 2D-HBT vdW heterostructure with different base thicknesses is investigated by two configurations, namely, common-emitter and common-base configurations. The 2D-HBT vdW heterostructure exhibited intriguing electrical characteristics of current amplification with large gains of α ≈ 1.11 and ß ≈ 20.7. In addition, 2D-HBT-based devices have been investigated as chemical sensors for the detection of NH3 and O2 gases at room temperature. The effects of different environments, such as air, vacuum, O2, and NH3, were also analyzed in dark conditions, and with a light of 633 nm wavelength, ultrahigh sensitivity and fast response and recovery times (6.55 and 16.2 ms, respectively) were observed. These unprecedented outcomes have huge potential in modern technology in the development of low-power amplifiers and gas sensors.

High performance complementary WS2 devices with hybrid Gr/Ni contacts.

Two-dimensional (2D) transition metal dichalcogenides have attracted vibrant interest for future solid-state device applications due to their unique properties. However, it is challenging to realize 2D material based high performance complementary devices due to the stubborn Fermi level pinning effect and the lack of facile doping techniques. In this paper, we reported a hybrid Gr/Ni contact to WS2, which can switch carrier types from n-type to p-type in WS2. The unorthodox polarity transition is attributed to the natural p-doping of graphene with Ni adsorption and the alleviation of Fermi level pinning in WS2. Furthermore, we realized asymmetric Ni and Gr/Ni hybrid contacts to a multilayer WS2 device, and we observed synergistic p-n diode characteristics with excellent current rectification exceeding 104, and a near unity ideality factor of 1.1 (1.6) at a temperature of 4.5 K (300 K). Lastly, our WS2 p-n device exhibits high performance photovoltaic ability with a maximum photoresponsivity of 4 × 104 A W-1 at 532 nm wavelength, that is 108 times higher than that of graphene and 50 times better than that of the monolayer MoS2 photodetector. This doping-free carrier type modulation technique will pave the way to realize high performance complementary electronics and optoelectronic devices based on 2D materials.

Asymmetric electrode incorporated 2D GeSe for self-biased and efficient photodetection.

2D layered germanium selenide (GeSe) with p-type conductivity is incorporated with asymmetric contact electrode of chromium/Gold (Cr/Au) and Palladium/Gold (Pd/Au) to design a self-biased, high speed and an efficient photodetector. The photoresponse under photovoltaic effect is investigated for the wavelengths of light (i.e. ~220, ~530 and ~850 nm). The device exhibited promising figures of merit required for efficient photodetection, specifically the Schottky barrier diode is highly sensitive to NIR light irradiation at zero voltage with good reproducibility, which is promising for the emergency application of fire detection and night vision. The high responsivity, detectivity, normalized photocurrent to dark current ratio (NPDR), noise equivalent power (NEP) and response time for illumination of light (~850 nm) are calculated to be 280 mA/W, 4.1 × 109 Jones, 3 × 107 W-1, 9.1 × 10-12 WHz-1/2 and 69 ms respectively. The obtained results suggested that p-GeSe is a novel candidate for SBD optoelectronics-based technologies.

Comparison of frictional forces on graphene and graphite.

We report on the frictional force between an SiN tip and graphene/graphite surfaces using lateral force microscopy. The cantilever we have used was made of an SiN membrane and has a low stiffness of 0.006 N m(-1). We prepared graphene flakes on a Si wafer covered with silicon oxides. The frictional force on graphene was smaller than that on the Si oxide and larger than that on graphite (multilayer of graphene). Force spectroscopy was also employed to study the van der Waals force between the graphene and the tip. Judging that the van der Waals force was also in graphite-graphene-silicon oxide order, the friction is suspected to be related to the van der Waals interactions. As the normal force acting on the surface was much weaker than the attractive force, such as the van der Waals force, the friction was independent of the normal force strength. The velocity dependency of the friction showed a logarithmic behavior which was attributed to the thermally activated stick-slip effect.