Cheol-Joo Kim

High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity

The large-scale growth of semiconducting thin films forms the basis of modern electronics and optoelectronics. A decrease in film thickness to the ultimate limit of the atomic, sub-nanometre length scale, a difficult limit for traditional semiconductors (such as Si and GaAs), would bring wide benefits for applications in ultrathin and flexible electronics, photovoltaics and display technology. For this, transition-metal dichalcogenides (TMDs), which can form stable three-atom-thick monolayers, provide ideal semiconducting materials with high electrical carrier mobility, and their large-scale growth on insulating substrates would enable the batch fabrication of atomically thin high-performance transistors and photodetectors on a technologically relevant scale without film transfer. In addition, their unique electronic band structures provide novel ways of enhancing the functionalities of such devices, including the large excitonic effect, bandgap modulation, indirect-to-direct bandgap transition, piezoelectricity and valleytronics. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high electrical performance remains an unsolved challenge. Here we report the preparation of high-mobility 4-inch wafer-scale films of monolayer molybdenum disulphide (MoS2) and tungsten disulphide, grown directly on insulating SiO2 substrates, with excellent spatial homogeneity over the entire films. They are grown with a newly developed, metal–organic chemical vapour deposition technique, and show high electrical performance, including an electron mobility of 30 cm2 V−1 s−1 at room temperature and 114 cm2 V−1 s−1 at 90 K for MoS2, with little dependence on position or channel length. With the use of these films we successfully demonstrate the wafer-scale batch fabrication of high-performance monolayer MoS2 field-effect transistors with a 99% device yield and the multi-level fabrication of vertically stacked transistor devices for three-dimensional circuitry. Our work is a step towards the realization of atomically thin integrated circuitry.

Graphene and boron nitride lateral heterostructures for atomically thin circuitry

Precise spatial control over the electrical properties of thin films is the key capability enabling the production of modern integrated circuitry. Although recent advances in chemical vapour deposition methods have enabled the large-scale production of both intrinsic and doped graphene, as well as hexagonal boron nitride (h-BN), controlled fabrication of lateral heterostructures in these truly atomically thin systems has not been achieved. Graphene/h-BN interfaces are of particular interest, because it is known that areas of different atomic compositions may coexist within continuous atomically thin films and that, with proper control, the bandgap and magnetic properties can be precisely engineered. However, previously reported approaches for controlling these interfaces have fundamental limitations and cannot be easily integrated with conventional lithography. Here we report a versatile and scalable process, which we call ‘patterned regrowth’, that allows for the spatially controlled synthesis of lateral junctions between electrically conductive graphene and insulating h-BN, as well as between intrinsic and substitutionally doped graphene. We demonstrate that the resulting films form mechanically continuous sheets across these heterojunctions. Conductance measurements confirm laterally insulating behaviour for h-BN regions, while the electrical behaviour of both doped and undoped graphene sheets maintain excellent properties, with low sheet resistances and high carrier mobilities. Our results represent an important step towards developing atomically thin integrated circuitry and enable the fabrication of electrically isolated active and passive elements embedded in continuous, one-atom-thick sheets, which could be manipulated and stacked to form complex devices at the ultimate thickness limit.

ELECTRICAL DETECTION OF VEGFS FOR CANCER DIAGNOSES USING ANTI-VASCULAR ENDOTHERIAL GROWTH FACTOR APTAMER-MODIFIED SI NANOWIRE FETS

We report the real-time, label-free and electrical detection of vascular endotherial growth factor (VEGF) for cancer diagnosis using anti-VEGF aptamer-modified Si nanowire field-effect transistors (SiNW-FETs). Specifically, the high quality single-crystalline SiNWs of both n-type and p-type characters were surface modified with the covalent immobilization of anti-VEGF aptamers, and they were turned into SiNW-FET biosensors for the VEGF detection. We show that the VEGF molecules consistently act on the gate dielectrics of both n-type and p-type SiNW-FETs as electrically positive point-charges; their recognition to anti-VEGF aptamers depletes (accumulates) the charge carriers in the p-type (n-type) SiNW-FETs and thus decreases (increases) the detection currents. The detection limit for VEGFs in this study was determined as 1.04nM and 104pM for the cases of n-type and p-type SiNW-FETs, respectively.

Diameter-Dependent Internal Gain in Ohmic Ge Nanowire Photodetectors

We report a diameter-dependent photoconduction gain in intrinsic Ge nanowire (NW) photodetectors. By employing a scanning photocurrent imaging technique, we provide evidence that the photocarrier transport is governed by the hole drift along the Ge NWs, ensuing the higher internal gain up to approximately 10(3) from the thin NWs. It is found that the magnitudes of both gain and photoconductivity are inversely proportional to the NW diameter ranging from 50 to 300 nm. We attribute our observations to the variation in the effective hole carrier density upon varying diameters of Ge NWs, as a result of field effects from the diameter-dependent population of the surface-trapped electrons, along with a model calculation. Our observations represent inherent size effects of internal gain in semiconductor NWs, thereby provide a new insight into nano-optoelectronics.

Polycrystalline Graphene with Single Crystalline Electronic Structure

We report the scalable growth of aligned graphene and hexagonal boron nitride on commercial copper foils, where each film originates from multiple nucleations yet exhibits a single orientation. Thorough characterization of our graphene reveals uniform crystallographic and electronic structures on length scales ranging from nanometers to tens of centimeters. As we demonstrate with artificial twisted graphene bilayers, these inexpensive and versatile films are ideal building blocks for large-scale layered heterostructures with angle-tunable optoelectronic properties.

Stacking Order Dependent Second Harmonic Generation and Topological Defects in h-BN Bilayers

The ability to control the stacking structure in layered materials could provide an exciting approach to tuning their optical and electronic properties. Because of the lower symmetry of each constituent monolayer, hexagonal boron nitride (h-BN) allows more structural variations in multiple layers than graphene; however, the structure-property relationships in this system remain largely unexplored. Here, we report a strong correlation between the interlayer stacking structures and optical and topological properties in chemically grown h-BN bilayers, measured mainly by using dark-field transmission electron microscopy (DF-TEM) and optical second harmonic generation (SHG) mapping. Our data show that there exist two distinct h-BN bilayer structures with different interlayer symmetries that give rise to a distinct difference in their SHG intensities. In particular, the SHG signal in h-BN bilayers is observed only for structures with broken inversion symmetry, with an intensity much larger than that of single layer h-BN. In addition, our DF-TEM data identify the formation of interlayer topological defects in h-BN bilayers, likely induced by local strain, whose properties are determined by the interlayer symmetry and the different interlayer potential landscapes.

The role of NiOx overlayers on spontaneous growth of NiSix nanowires from Ni seed layers.

We report a controllably reproducible and spontaneous growth of single-crystalline NiSix nanowires using NiOx/Ni seed layers during SiH4 chemical vapor deposition (CVD). We provide evidence that upon the reactions of SiH4 (vapor)-Ni seed layers (solid), the presence of the NiOx overlayer on Ni seed layers plays the key role to promote the spontaneous one-dimensional growth of NiSix single crystals without employing catalytic nanocrystals. Specifically, the spontaneous nanowire formation on the NiOx overlayer is understood within the frame of the SiH4 vapor-phase reaction with out-diffused Ni from the Ni underlayers, where the Ni diffusion is controlled by the NiOx overlayers for the limited nucleation. We show that single-crystalline NiSix nanowires by this self-organized fashion in our synthesis display a narrow diameter distribution, and their average length is set by the thickness of the Ni seed layers. We argue that our simple CVD method employing the bilayers of transition metal and their oxides as the seed layers can provide implication as the general synthetic route for the spontaneous growth of metal-silicide nanowires in large scales.

Atomically Thin Ohmic Edge Contacts Between Two-Dimensional Materials

With the decrease of the dimensions of electronic devices, the role played by electrical contacts is ever increasing, eventually coming to dominate the overall device volume and total resistance. This is especially problematic for monolayers of semiconducting transition-metal dichalcogenides (TMDs), which are promising candidates for atomically thin electronics. Ideal electrical contacts to them would require the use of similarly thin electrode materials while maintaining low contact resistances. Here we report a scalable method to fabricate ohmic graphene edge contacts to two representative monolayer TMDs, MoS2 and WS2. The graphene and TMD layer are laterally connected with wafer-scale homogeneity, no observable overlap or gap, and a low average contact resistance of 30 kΩ·μm. The resulting graphene edge contacts show linear current-voltage (I-V) characteristics at room temperature, with ohmic behavior maintained down to liquid helium temperatures.

Chiral atomically thin films

Chiral materials possess left- and right-handed counterparts linked by mirror symmetry. These materials are useful for advanced applications in polarization optics, stereochemistry and spintronics. In particular, the realization of spatially uniform chiral films with atomic-scale control of their handedness could provide a powerful means for developing nanodevices with novel chiral properties. However, previous approaches based on natural or grown films, or arrays of fabricated building blocks, could not offer a direct means to program intrinsic chiral properties of the film on the atomic scale. Here, we report a chiral stacking approach, where two-dimensional materials are positioned layer-by-layer with precise control of the interlayer rotation (θ) and polarity, resulting in tunable chiral properties of the final stack. Using this method, we produce left- and right-handed bilayer graphene, that is, a two-atom-thick chiral film. The film displays one of the highest intrinsic ellipticity values (6.5 deg μm(-1)) ever reported, and a remarkably strong circular dichroism (CD) with the peak energy and sign tuned by θ and polarity. We show that these chiral properties originate from the large in-plane magnetic moment associated with the interlayer optical transition. Furthermore, we show that we can program the chiral properties of atomically thin films layer-by-layer by producing three-layer graphene films with structurally controlled CD spectra.

Hyperspectral Imaging of Structure and Composition in Atomically Thin Heterostructures

Precise vertical stacking and lateral stitching of two-dimensional (2D) materials, such as graphene and hexagonal boron nitride (h-BN), can be used to create ultrathin heterostructures with complex functionalities, but this diversity of behaviors also makes these new materials difficult to characterize. We report a DUV-vis-NIR hyperspectral microscope that provides imaging and spectroscopy at energies of up to 6.2 eV, allowing comprehensive, all-optical mapping of chemical composition in graphene/h-BN lateral heterojunctions and interlayer rotations in twisted bilayer graphene (tBLG). With the addition of transmission electron microscopy, we obtain quantitative structure-property relationships, confirming the formation of interfaces in graphene/h-BN lateral heterojunctions that are abrupt on a micrometer scale, and a one-to-one relationship between twist angle and interlayer optical resonances in tBLG. Furthermore, we perform similar hyperspectral imaging of samples that are supported on a nontransparent silicon/SiO2 substrate, enabling facile fabrication of atomically thin heterostructure devices with known composition and structure.