Highly Efficient Van Der Waals Heterojunction on Graphdiyne toward the High-Performance Photodetector.

Graphdiyne (GDY), a new 2D material, has recently proven excellent performance in photodetector applications due to its direct bandgap and high mobility. Different from the zero-gap of graphene, these preeminent properties made GDY emerge as a rising star for solving the bottleneck of graphene-based inefficient heterojunction. Herein, a highly effective graphdiyne/molybdenum (GDY/MoS2 ) type-II heterojunction in a charge separation is reported toward a high-performance photodetector. Characterized by robust electron repulsion of alkyne-rich skeleton, the GDY based junction facilitates the effective electron-hole pairs separation and transfer. This results in significant suppression of Auger recombination up to six times at the GDY/MoS2 interface compared with the pristine materials owing to an ultrafast hot hole transfer from MoS2 to GDY. GDY/MoS2 device demonstrates notable photovoltaic behavior with a short-circuit current of -1.3 × 10-5 A and a large open-circuit voltage of 0.23 V under visible irradiation. As a positive-charge-attracting magnet, under illumination, alkyne-rich framework induces positive photogating effect on the neighboring MoS2 , further enhancing photocurrent. Consequently, the device exhibits broadband detection (453-1064 nm) with a maximum responsivity of 78.5 A W-1 and a high speed of 50 µs. Results open up a new promising strategy using GDY toward effective junction for future optoelectronic applications.

CNT-molecule-CNT (1D-0D-1D) van der Waals integration ferroelectric memory with 1-nm2 junction area.

The device's integration of molecular electronics is limited regarding the large-scale fabrication of gap electrodes on a molecular scale. The van der Waals integration (vdWI) of a vertically aligned molecular layer (0D) with 2D or 3D electrodes indicates the possibility of device's integration; however, the active junction area of 0D-2D and 0D-3D vdWIs remains at a microscale size. Here, we introduce the robust fabrication of a vertical 1D-0D-1D vdWI device with the ultra-small junction area of 1 nm2 achieved by cross-stacking top carbon nanotubes (CNTs) on molecularly assembled bottom CNTs. 1D-0D-1D vdWI memories are demonstrated through ferroelectric switching of azobenzene molecules owing to the cis-trans transformation combined with the permanent dipole moment of the end-tail -CF3 group. In this work, our 1D-0D-1D vdWI memory exhibits a retention performance above 2000 s, over 300 cycles with an on/off ratio of approximately 105 and record current density (3.4 × 108 A/cm2), which is 100 times higher than previous study through the smallest junction area achieved in a vdWI. The simple stacking of aligned CNTs (4 × 4) allows integration of memory arrays (16 junctions) with high device operational yield (100%), offering integration guidelines for future molecular electronics.

Synthesis of a Selectively Nb-Doped WS2-MoS2 Lateral Heterostructure for a High-Detectivity PN Photodiode.

In this study, selective Nb doping (P-type) at the WS2 layer in a WS2-MoS2 lateral heterostructure via a chemical vapor deposition (CVD) method using a solution-phase precursor containing W, Mo, and Nb atoms is proposed. The different chemical activity reactivity (MoO3 > WO3 > Nb2O5) enable the separation of the growth temperature of intrinsic MoS2 to 700 °C (first grown inner layer) and Nb-doped WS2 to 800 °C (second grown outer layer). By controlling the Nb/(W+Nb) molar ratio in the solution precursor, the hole carrier density in the p-type WS2 layer is selectively controlled from approximately 1.87 × 107/cm2 at 1.5 at.% Nb to approximately 1.16 × 1013/cm2 at 8.1 at.% Nb, while the electron carrier density in n-type MoS2 shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS2-MoS2 heterostructure transforms from the N-N junction (0 at.% Nb) to the P-N junction (4.5 at.% Nb) and the P-N tunnel junction (8.1 at.% Nb). The band-to-band tunneling at the P-N tunnel junction (8.1 at.% Nb) is eliminated by applying negative gate bias, resulting in a maximum rectification ratio (105) and a minimum channel resistance (108 Ω). With this optimized photodiode (8.1 at.% Nb at Vg = -30 V), an Iphoto/Idark ratio of 6000 and a detectivity of 1.1 × 1014 Jones are achieved, which are approximately 20 and 3 times higher, respectively, than the previously reported highest values for CVD-grown transition-metal dichalcogenide P-N junctions.

One-Step Synthesis of NbSe2/Nb-Doped-WSe2 Metal/Doped-Semiconductor van der Waals Heterostructures for Doping Controlled Ohmic Contact.

van der Waals heterostructures (vdWHs) of metallic (m-) and semiconducting (s-) transition-metal dichalcogenides (TMDs) exhibit an ideal metal/semiconductor (M/S) contact in a field-effect transistor. However, in the current two-step chemical vapor deposition process, the synthesis of m-TMD on pregrown s-TMD contaminates the van der Waals (vdW) interface and hinders the doping of s-TMD. Here, NbSe2/Nb-doped-WSe2 metal-doped-semiconductor (M/d-S) vdWHs are created via a one-step synthesis approach using a niobium molar ratio-controlled solution-phase precursor. The one-step growth approach synthesizes Nb-doped WSe2 with a controllable doping concentration and metal/doped-semiconductor vdWHs. The hole carrier concentration can be precisely controlled by controlling the Nb/(W + Nb) molar ratio in the precursor solution from ∼3 × 1011/cm2 at Nb-0% to ∼1.38 × 1012/cm2 at Nb-60%; correspondingly, the contact resistance RC value decreases from 10 888.78 at Nb-0% to 70.60 kΩ.µm at Nb-60%. The Schottky barrier height measurement in the Arrhenius plots of ln(Isat/T2) versus q/KBT demonstrated an ohmic contact in the NbSe2/WxNb1-xSe2 vdWHs. Combining p-doping in WSe2 and M/d-S vdWHs, the mobility (27.24 cm2 V-1 s-1) and on/off ratio (2.2 × 107) are increased 1238 and 4400 times, respectively, compared to that using the Cr/pure-WSe2 contact (0.022 cm2 V-1 s-1 and 5 × 103, respectively). Together, the RC value using the NbSe2 contact shows 2.46 kΩ.µm, which is ∼29 times lower than that of using a metal contact. This method is expected to guide the synthesis of various M/d-S vdWHs and applications in future high-performance integrated circuits.

Selective Pattern Growth of Atomically Thin MoSe2 Films via a Surface-Mediated Liquid-Phase Promoter.

Two-dimensional transition metal dichalcogenides (TMDs) offer numerous advantages over silicon-based application in terms of atomically thin geometry, excellent opto-electrical properties, layer-number dependence, band gap variability, and lack of dangling bonds. The production of high-quality and large-scale TMD films is required with consideration of practical technology. However, the performance of scalable devices is affected by problems such as contamination and patterning arising from device processing; this is followed by an etching step, which normally damages the TMD film. Herein, we report the direct growth of MoSe2 films on selective pattern areas via a surface-mediated liquid-phase promoter using a solution-based approach. Our growth process utilizes the promoter on the selective pattern area by enhancing wettability, resulting in a highly uniform MoSe2 film. Moreover, our approach can produce other TMD films such as WSe2 films as well as control various pattern shapes, sizes, and large-scale areas, thus improving their applicability in various devices in the future. Our patterned MoSe2 field-effect transistor device exhibits a p-type dominant conduction behavior with a high on/off current ratio of ∼106. Thus, our study provides general guidance for direct selective pattern growth via a solution-based approach and the future design of integrated devices for a large-scale application.

Ferromagnetic Order at Room Temperature in Monolayer WSe2 Semiconductor via Vanadium Dopant.

Diluted magnetic semiconductors including Mn-doped GaAs are attractive for gate-controlled spintronics but Curie transition at room temperature with long-range ferromagnetic order is still debatable to date. Here, the room-temperature ferromagnetic domains with long-range order in semiconducting V-doped WSe2 monolayer synthesized by chemical vapor deposition are reported. Ferromagnetic order is manifested using magnetic force microscopy up to 360 K, while retaining high on/off current ratio of ≈105 at 0.1% V-doping concentration. The V-substitution to W sites keeps a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution scanning transmission electron microscopy. More importantly, the ferromagnetic order is clearly modulated by applying a back-gate bias. The findings open new opportunities for using 2D transition metal dichalcogenides for future spintronics.

CVD-Grown Carbon Nanotube Branches on Black Silicon Stems for Ultrahigh Absorbance in Wide Wavelength Range.

We report a black silicon-carbon nanotube (bSi-CNT) hybrid structure for ultrahigh absorbance at wide spectral range of wavelength (300-1200 nm). CNTs are densely grown on entire bSi stems by chemical vapor deposition (CVD) through uniformly coating Fe catalyst. The bSi-CNT not only increases the surface roughness for enhancing the light suppression, but also allows the absorption of light in a wide wavelength range over the Si band gap (>1000 nm owing to 1.1 eV) due to the small band gap of CNT (0.6 eV). At short wavelength below Si band gap (<1000 nm), the absorbance of bSi-CNT shows average of 98.1%, while bSi shows 89.4%, which is because of high surface roughness of bSi-CNT that enhancing the light trapping. At long wavelength over Si band gap, the absorbance of bSi-CNT was maintained to 96.3% because of the absorption in CNT, while absorbance of bSi abruptly reduces with increase wavelength. Especially, the absorbance of bSi-CNT was showed 93.5% at 1200 nm, which is about 30~90% higher than previously reported bSi. Simple growth of CNTs on bSi can dramatically enhances the absorbance without using any antireflection coating layer. Thus, this study can be employed for realizing high efficiency photovoltaic, photocatalytic applications.

Unveiling the Hot Carrier Distribution in Vertical Graphene/h-BN/Au van der Waals Heterostructures for High-Performance Photodetector.

Graphene is one of the most promising materials for photodetectors due to its ability to convert photons into hot carriers within approximately 50 fs and generate long-lived thermalized states with lifetimes longer than 1 ps. In this study, we demonstrate a wide range of vertical photodetectors having a graphene/h-BN/Au heterostructure in which an hexagonal boron nitride (h-BN) insulating layer is inserted between an Au electrode and graphene photoabsorber. The photocarriers effectively tunnel through the small hole barrier (1.93 eV) at the Au/h-BN junction while the dark carriers are highly suppressed by a large electron barrier (2.27 eV) at the graphene/h-BN junction. Thus, an extremely low dark current of ∼10-13 A is achieved, which is 8 orders of magnitude lower than that of graphene lateral photodetector devices (∼10-5 A). Also, our device displays an asymmetric photoresponse behavior due to photothermionic emission at the graphene/h-BN and Au/h-BN junctions. The asymmetric behavior generates additional thermal carriers (hot carriers) to enable our device to generate photocurrents that can overcome the Schottky barrier. Furthermore, our device shows the highest value of the Iph/Idark ratio of ∼225 at 7 nm thick h-BN insulating layer, which is 3 orders of magnitude larger than that of the previously reported graphene lateral photodetectors without any active materials. In addition, we achieve a fast response speed of 12 µs of rise time and 5 µs of fall time, which are about 100 times faster than those of other graphene integrated photodetectors.

Tunable Negative Differential Resistance in van der Waals Heterostructures at Room Temperature by Tailoring the Interface.

Vertically stacked two-dimensional van der Waals (vdW) heterostructures, used to obtain homogeneity and band steepness at interfaces, exhibit promising performance for band-to-band tunneling (BTBT) devices. Esaki tunnel diodes based on vdW heterostructures, however, yield poor current density and peak-to-valley ratio, inferior to those of three-dimensional materials. Here, we report the negative differential resistance (NDR) behavior in a WSe2/SnSe2 heterostructure system at room temperature and demonstrate that heterointerface control is one of the keys to achieving high device performance by constructing WSe2/SnSe2 heterostructures in inert gas environments. While devices fabricated in ambient conditions show poor device performance due to the observed oxidation layer at the interface, devices fabricated in inert gas exhibit extremely high peak current density up to 1460 mA/mm2, 3-4 orders of magnitude higher than reported vdW heterostructure-based tunnel diodes, with a peak-to-valley ratio of more than 4 at room temperature. Besides, Pd/WSe2 contact in our device possesses a much higher Schottky barrier than previously reported Cr/WSe2 contact in the WSe2/SnSe2 device, which suppresses the thermionic emission current to less than the BTBT current level, enabling the observation of NDR at room temperature. Diode behavior can be further modulated by controlling the electrostatic doping and the tunneling barrier as well.

Efficient Gate Modulation in a Screening-Engineered MoS2/Single-Walled Carbon Nanotube Network Heterojunction Vertical Field-Effect Transistor.

In this report, a screening-engineered carbon nanotube (CNT) network/MoS2/metal heterojunction vertical field effect transistor (CNT-VFET) is fabricated for an efficient gate modulation independent of the drain voltage. The gate field in the CNT-VFET transports through the empty space of the CNT network without any screening layer and directly modulates the MoS2 semiconductor energy band, while the gate field from the Si back gate is mostly screened by the graphene layer. Consequently, the on/off ratio of CNT-VFET maintained 103 in overall drain voltages, which is 10 times and 1000 times higher than that of the graphene (Gr) VFET at Vsd = 0.1 (ratio = 81.9) and 1 V (ratio = 3), respectively. An energy band diagram simulation shows that the Schottky barrier modulation of CNT/MoS2 contact along the sweeping gate bias is independent of the drain voltage. On the other hand, the gate modulation of Gr/MoS2 is considerably reduced with increased drain voltage because more electrons are drawn into the graphene electrode and screens the gate field by applying a higher drain voltage to the graphene/MoS2/metal capacitor.