Robin B. Jacobs-Gedrim

Extraordinary Photoresponse in Two-Dimensional In2Se3 Nanosheets

We demonstrate extraordinary photoconductive behavior in two-dimensional (2D) crystalline indium selenide (In2Se3) nanosheets. Photocurrent measurements reveal that semiconducting In2Se3 nanosheets have an extremely high response to visible light, exhibiting a photoresponsivity of 3.95 × 10(2) A·W(-1) at 300 nm with an external quantum efficiency greater than 1.63 × 10(5) % at 5 V bias. The key figures-of-merit exceed that of graphene and other 2D material-based photodetectors reported to date. In addition, the photodetector has a fast response time of 1.8 × 10(-2) s and a specific detectivity of 2.26 × 10(12) Jones. The photoconductive response of α-In2Se3 nanosheets extends into ultraviolet, visible, and near-infrared spectral regions. The high photocurrent response is attributed to the direct band gap (EG = 1.3 eV) of In2Se3 combined with a large surface-area-to-volume ratio and a self-terminated/native-oxide-free surface, which help to reduce carrier recombination while keeping fast response, allowing for real-time detection under very low-light conditions.

Defect-Induced Photoluminescence in Monolayer Semiconducting Transition Metal Dichalcogenides

It is well established that defects strongly influence properties in two-dimensional materials. For graphene, atomic defects activate the Raman-active centrosymmetric A1g ring-breathing mode known as the D-peak. The relative intensity of this D-peak compared to the G-band peak is the most widely accepted measure of the quality of graphene films. However, no such metric exists for monolayer semiconducting transition metal dichalcogenides such as WS2 or MoS2. Here we intentionally create atomic-scale defects in the hexagonal lattice of pristine WS2 and MoS2 monolayers using plasma treatments and study the evolution of their Raman and photoluminescence spectra. High-resolution transmission electron microscopy confirms plasma-induced creation of atomic-scale point defects in the monolayer sheets. We find that while the Raman spectra of semiconducting transition metal dichalcogenides (at 532 nm excitation) are insensitive to defects, their photoluminescence reveals a distinct defect-related spectral feature located ∼0.1 eV below the neutral free A-exciton peak. This peak originates from defect-bound neutral excitons and intensifies as the two-dimensional (2D) sheet is made more defective. This spectral feature is observable in air under ambient conditions (room temperature and atmospheric pressure), which allows for a relatively simple way to determine the defectiveness of 2D semiconducting nanosheets. Controlled defect creation could also enable tailoring of the optical properties of these materials in optoelectronic device applications.

2D layered insulator hexagonal boron nitride enabled surface passivation in dye sensitized solar cells

A two-dimensional layered insulator, hexagonal boron nitride (h-BN), is demonstrated as a new class of surface passivation materials in dye-sensitized solar cells (DSSCs) to reduce interfacial carrier recombination. We observe ~57% enhancement in the photo-conversion efficiency of the DSSC utilizing h-BN coated semiconductor TiO2 as compared with the device without surface passivation. The h-BN coated TiO2 is characterized by Raman spectroscopy to confirm the presence of highly crystalline, mixed monolayer/few-layer h-BN nanoflakes on the surface of TiO2. The passivation helps to minimize electron-hole recombination at the TiO2/dye/electrolyte interfaces. The DSSC with h-BN passivation exhibits significantly lower dark saturation current in the low forward bias region and higher saturation in the high forward bias region, respectively, suggesting that the interface quality is largely improved without impeding carrier transport at the material interface. The experimental results reveal that the emerging 2D layered insulator could be used for effective surface passivation in solar cell applications attributed to desirable material features such as high crystallinity and self-terminated/dangling-bond-free atomic planes as compared with high-k thin-film dielectrics.

Exploring carrier transport phenomena in a CVD-assembled graphene FET on hexagonal boron nitride

The supporting substrate plays a crucial role in preserving the superb electrical characteristicsof an atomically thin 2D carbon system. We explore carrier transport behavior in achemical-vapor-deposition- (CVD-) assembled graphene monolayer on hexagonal boron nitride (h-BN) substrate. Graphene-channel field-effect transistors (GFETs) were fabricated on ultra-thin h-BN multilayers to screen out carrier scattering from the underlying SiO2 substrate. To explore the transport phenomena, we use three different approaches to extract carrier mobility, namely, effective carrier mobility (μFE), intrinsic carrier mobility (μ), and field-effect mobility (μFE). A comparative study has been conducted based on the electrical characterization results, uncovering the impacts of supporting substrate material and device geometry scaling on carrier mobility in GFETs with CVD-assembled graphene as the active channel.

Graphene interconnects fully encapsulated in layered insulator hexagonal boron nitride

We demonstrate improvements in the electrical performance of graphene interconnects with full encapsulation by lattice-matching layered insulator, hexagonal boron nitride (h-BN). A novel layer-based transfer method is developed to assemble the top passivating layer of h-BN on the graphene surface to construct the h-BN/graphene/h-BN heterostructures. The encapsulated graphene interconnects (EGIs) are characterized and compared with graphene interconnects on either SiO₂ or h-BN substrates with no top passivating h-BN layer. We observe significant improvements in both the maximum current density and breakdown voltage in EGIs. Compared with the uncovered structures, EGIs also show an appreciable increase (∼67%) in power density at breakdown. These improvements are achieved without degrading the carrier transport characteristics in graphene wires. In addition, EGIs exhibit a minimal environment impact, showing electrical behavior insensitive to ambient conditions.

Electrical Conduction and Reliability in Dual-Layered Graphene Heterostructure Interconnects

Dual-layer graphene (DLG) interconnects with hexagonal boron nitride (h-BN) as intercalated insulating layer have been demonstrated. The DLG employs graphene grown by chemical vapor deposition process with h-BN serving as a barrier preventing interlayer scattering, which degrades carrier transport in multilayer graphene. The conductive behavior in dual-layer structures is compared with monolayer graphene and randomly stacked bilayer graphene. Reduced resistance is observed in DLG, which exhibits higher current-carrying capacity and maximum power density. In addition, DLG wire is shown to be robust under constant voltage stressing (10 V) at an elevated temperature (150°C) with the mean time to failure ~75 times higher than that of stacked bilayer graphene wires.

Chemical assembly and electrical characteristics of surface-rich topological insulator Bi2Se3 nanoplates and nanoribbons

We demonstrate synthesis of low-dimensional, surface-rich bismuth selenide nanoplates and nanoribbons through a low-pressure chemical-vapor-deposition method. The single crystalline lattice structure, morphology, and chemical composition of the synthesized nanoplates and nanoribbons are analyzed. As-prepared samples are found to be all n-type doped. Very large surface-to-volume ratios have been achieved in these low-dimensional nanostructures, making them ideal for investigating topological insulator properties. Gate-controlled bismuth selenide nanoplate field-effect transistors are fabricated and basic electrical behavior is characterized.

Scalable synthesis of two-dimensional antimony telluride nanoplates down to a single quintuple layer

Scalable syntheses of two-dimensional topological insulators are critical to material exploration. We demonstrate a controlled assembly of a two-dimensional V–VI group compound, Sb2Te3 nanoplates (NPs), through a vapor–solid growth process. The physical thickness of Sb2Te3 NPs can be rationally controlled in a wide range, from hundreds of nm down to sub-10 nm. Single-quintuple-layer Sb2Te3 NPs were obtained, with a high domain density of ∼2.465 × 108 cm−2 over a large surface area (1 cm × 1 cm) of a SiO2/Si substrate, verifying a scalable synthesis method. Extensive material analyses were conducted to explore the basic properties of Sb2Te3 NPs using SEM and AFM, etc. HRTEM analysis confirms that the NP samples exhibit a highly crystalline structure and XPS analysis confirms the chemical composition and material stoichiometry. The growth of 2D topological insulator nanostructures may open up new opportunities in surface-state studies and potential applications in low-dissipative electronic systems.

Layered insulator hexagonal boron nitride for surface passivation in quantum dot solar cell

Single crystalline, two dimensional (2D) layered insulator hexagonal boron nitride (h-BN), is demonstrated as an emerging material candidate for surface passivation on mesoporous TiO2. Cadmium selenide (CdSe) quantum dot based bulk heterojunction (BHJ) solar cell employed h-BN passivated TiO2 as an electron acceptor exhibits photoconversion efficiency ∼46% more than BHJ employed unpassivated TiO2. Dominant interfacial recombination pathways such as electron capture by TiO2 surface states and recombination with hole at valence band of CdSe are efficiently controlled by h-BN enabled surface passivation, leading to improved photovoltaic performance. Highly crystalline, confirmed by transmission electron microscopy, dangling bond-free 2D layered h-BN with self-terminated atomic planes, achieved by chemical exfoliation, enables efficient passivation on TiO2, allowing electronic transport at TiO2/h-BN/CdSe interface with much lower recombination rate compared to an unpassivated TiO2/CdSe interface.

Reversible phase-change behavior in two-dimensional antimony telluride (Sb2Te3) nanosheets

Potential two-dimensional (2D) van der Waals crystals with mechanical flexibility, transparency, and low cost are viable material platforms for future nanodevices. Resistive switching behavior in 2D layered Sb2Te3 nanosheets is demonstrated. Nearly three orders of magnitude switch in sheet resistance were realized for more than 20 cycles. The observed hysteretic behavior is due to the change between crystalline and amorphous phases under a melt-quench-recrystallization mechanism. More importantly, the energy required to amorphize the nanosheets decreases exponentially with layer thickness reduction. It is expected that scaling to the ultimate two-dimensional limit in chalcogenide nanosheet-based phase change devices may meet or even exceed the energy efficiency of neurobiological architectures.