Keji Lai

Aharonov–Bohm interference in topological insulator nanoribbons

Topological insulators represent unusual phases of quantum matter with an insulating bulk gap and gapless edges or surface states. The two-dimensional topological insulator phase was predicted in HgTe quantum wells and confirmed by transport measurements. Recently, Bi(2)Se(3) and related materials have been proposed as three-dimensional topological insulators with a single Dirac cone on the surface, protected by time-reversal symmetry. The topological surface states have been observed by angle-resolved photoemission spectroscopy experiments. However, few transport measurements in this context have been reported, presumably owing to the predominance of bulk carriers from crystal defects or thermal excitations. Here we show unambiguous transport evidence of topological surface states through periodic quantum interference effects in layered single-crystalline Bi(2)Se(3) nanoribbons, which have larger surface-to-volume ratios than bulk materials and can therefore manifest surface effects. Pronounced Aharonov-Bohm oscillations in the magnetoresistance clearly demonstrate the coherent propagation of two-dimensional electrons around the perimeter of the nanoribbon surface, as expected from the topological nature of the surface states. The dominance of the primary h/e oscillation, where h is Plancks constant and e is the electron charge, and its temperature dependence demonstrate the robustness of these states. Our results suggest that topological insulator nanoribbons afford promising materials for future spintronic devices at room temperature.

Ambipolar field effect in the ternary topological insulator (BixSb1-x)2Te3 by composition tuning

Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping, and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3, has been explored extensively by means of material doping and electrical gating, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (Bi(x)Sb(1-x))2Te3 is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (Bi(x)Sb(1-x))2Te3 nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.

Topological Insulator Nanowires and Nanoribbons

Recent theoretical calculations and photoemission spectroscopy measurements on the bulk Bi(2)Se(3) material show that it is a three-dimensional topological insulator possessing conductive surface states with nondegenerate spins, attractive for dissipationless electronics and spintronics applications. Nanoscale topological insulator materials have a large surface-to-volume ratio that can manifest the conductive surface states and are promising candidates for devices. Here we report the synthesis and characterization of high quality single crystalline Bi(2)Se(3) nanomaterials with a variety of morphologies. The synthesis of Bi(2)Se(3) nanowires and nanoribbons employs Au-catalyzed vapor-liquid-solid (VLS) mechanism. Nanowires, which exhibit rough surfaces, are formed by stacking nanoplatelets along the axial direction of the wires. Nanoribbons are grown along [1120] direction with a rectangular cross-section and have diverse morphologies, including quasi-one-dimensional, sheetlike, zigzag and sawtooth shapes. Scanning tunneling microscopy (STM) studies on nanoribbons show atomically smooth surfaces with approximately 1 nm step edges, indicating single Se-Bi-Se-Bi-Se quintuple layers. STM measurements reveal a honeycomb atomic lattice, suggesting that the STM tip couples not only to the top Se atomic layer, but also to the Bi atomic layer underneath, which opens up the possibility to investigate the contribution of different atomic orbitals to the topological surface states. Transport measurements of a single nanoribbon device (four terminal resistance and Hall resistance) show great promise for nanoribbons as candidates to study topological surface states.

Toward air-stable multilayer phosphorene thin-films and transistors

Few-layer black phosphorus (BP), also known as phosphorene, is poised to be the most attractive graphene analogue owing to its high mobility approaching that of graphene, and its thickness-tunable band gap that can be as large as that of molybdenum disulfide. In essence, phosphorene represents the much sought after high-mobility, large direct band gap two-dimensional layered crystal that is ideal for optoelectronics and flexible devices. However, its instability in air is of paramount concern for practical applications. Here, we demonstrate air-stable BP devices with dielectric and hydrophobic encapsulation. Microscopy, spectroscopy, and transport techniques were employed to elucidate the aging mechanism, which can initiate from the BP surface for bare samples, or edges for samples with thin dielectric coating, highlighting the ineffectiveness of conventional scaled dielectrics. Our months-long studies indicate that a double layer capping of Al2O3 and hydrophobic fluoropolymer affords BP devices and transistors with indefinite air-stability for the first time, overcoming a critical material challenge for applied research and development.

Rapid Surface Oxidation as a Source of Surface Degradation Factor for Bi2Se3

Bismuth selenide (Bi(2)Se(3)) is a topological insulator with metallic surface states (SS) residing in a large bulk bandgap. In experiments, synthesized Bi(2)Se(3) is often heavily n-type doped due to selenium vacancies. Furthermore, it is discovered from experiments on bulk single crystals that Bi(2)Se(3) gets additional n-type doping after exposure to the atmosphere, thereby reducing the relative contribution of SS in total conductivity. In this article, transport measurements on Bi(2)Se(3) nanoribbons provide additional evidence of such environmental doping process. Systematic surface composition analyses by X-ray photoelectron spectroscopy reveal fast formation and continuous growth of native oxide on Bi(2)Se(3) under ambient conditions. In addition to n-type doping at the surface, such surface oxidation is likely the material origin of the degradation of topological SS. Appropriate surface passivation or encapsulation may be required to probe topological SS of Bi(2)Se(3) by transport measurements.

Weak Antilocalization in Bi2(SexTe1–x)3 Nanoribbons and Nanoplates

Studying the surface states of Bi(2)Se(3) and Bi(2)Te(3) topological insulators has proven challenging due to the high bulk carrier density that masks the surface states. Ternary compound Bi(2)(Se(x)Te(1-x))(3) may present a solution to the current materials challenge by lowering the bulk carrier mobility significantly. Here, we synthesized Bi(2)(Se(x)Te(1-x))(3) nanoribbons and nanoplates via vapor-liquid-solid and vapor-solid growth methods where the atomic ratio x was controlled by the molecular ratio of Bi(2)Se(3) to Bi(2)Te(3) in the source mixture and ranged between 0 and 1. For the whole range of x, the ternary nanostructures are single crystalline without phase segregation, and their carrier densities decrease with x. However, the lowest electron density is still high (~10(19) cm(-3)) and the mobility low, suggesting that the majority of these carriers may come from impurity states. Despite the high carrier density, weak antilocalization (WAL) is clearly observed. Angle-dependent magnetoconductance study shows that an appropriate magnetic field range is critical to capture a true, two-dimensional (2D) WAL effect, and a fit to the 2D localization theory gives α of -0.97, suggesting its origin may be the topological surface states. The power law dependence of the dephasing length on temperature is ~T(-0.49) within the appropriate field range (~0.3 T), again reflecting the 2D nature of the WAL. Careful analysis on WAL shows how the surface states and the bulk/impurity states may interact with each other.

Mesoscopic Percolating Resistance Network in a Strained Manganite Thin Film

Separating Under Strain Complex oxides, such as cuprate superconductors and perovskites, often exhibit microscopic phase separation, where two or more phases coexist on the macroscopic scale but are spatially separated on the microscopic scale. Lai et al. (p. 190) studied a configuration often found in technological applications, a strained manganite thin film placed on a substrate. Microwave impedance microscopy, which differentiates between conducting and insulating areas on the thin film, allowed visualization of the phase separation as the magnetic field was varied. A network of conducting domains was observed whose orientation and characteristic length scales suggest that the substrate-exerted strain was involved in network formation. Microwave microscopy is used to image the phase separation of spin- and charge-ordered domains in magnetic thin films. Many unusual behaviors in complex oxides are deeply associated with the spontaneous emergence of microscopic phase separation. Depending on the underlying mechanism, the competing phases can form ordered or random patterns at vastly different length scales. By using a microwave impedance microscope, we observed an orientation-ordered percolating network in strained Nd1/2Sr1/2MnO3 thin films with a large period of 100 nanometers. The filamentary metallic domains align preferentially along certain crystal axes of the substrate, suggesting the anisotropic elastic strain as the key interaction in this system. The local impedance maps provide microscopic electrical information of the hysteretic behavior in strained thin film manganites, suggesting close connection between the glassy order and the colossal magnetoresistance effects at low temperatures.Keji Lai, Masao Nakamura, Worasom Kundhikanjana, Masashi Kawasaki, Yoshinori Tokura, 4 Michael A. Kelly, and Zhi-Xun Shen Department of Applied Physics and Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305 Cross-Correlated Materials Group (CMRG), ASI, RIKEN, Wako, 351-0198, Japan WPI-Advanced Institute for Materials Research (AIMR), Tohoku University, Sendai, 980-8577, Japan Department of Applied Physics, University of Tokyo, Tokyo 113-8586, Japan (Dated: July 19, 2010)

Ultrathin topological insulator Bi2Se3 nanoribbons exfoliated by atomic force microscopy.

Ultrathin topological insulator nanostructures, in which coupling between top and bottom surface states takes place, are of great intellectual and practical importance. Due to the weak van der Waals interaction between adjacent quintuple layers (QLs), the layered bismuth selenide (Bi(2)Se(3)), a single Dirac-cone topological insulator with a large bulk gap, can be exfoliated down to a few QLs. In this paper, we report the first controlled mechanical exfoliation of Bi(2)Se(3) nanoribbons (>50 QLs) by an atomic force microscope (AFM) tip down to a single QL. Microwave impedance microscopy is employed to map out the local conductivity of such ultrathin nanoribbons, showing drastic difference in sheet resistance between 1-2 QLs and 4-5 QLs. Transport measurement carried out on an exfoliated (<or=5 QLs) Bi(2)Se(3) device shows nonmetallic temperature dependence of resistance, in sharp contrast to the metallic behavior seen in thick (>50 QLs) ribbons. These AFM-exfoliated thin nanoribbons afford interesting candidates for studying the transition from quantum spin Hall surface to edge states.

Modeling and characterization of a cantilever-based near-field scanning microwave impedance microscope

This paper presents a detailed modeling and characterization of a microfabricated cantilever-based scanning microwave probe with separated excitation and sensing electrodes. Using finite-element analysis, we model the tip-sample interaction as small impedance changes between the tip electrode and the ground at our working frequencies near 1 GHz. The equivalent lumped elements of the cantilever can be determined by transmission line simulation of the matching network, which routes the cantilever signals to 50 Omega feed lines. In the microwave electronics, the background common-mode signal is canceled before the amplifier stage so that high sensitivity (below 1 aF capacitance changes) is obtained. Experimental characterization of the microwave microscope was performed on ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or topographical signals can be obtained from different reflection modes of the probe.We present a detailed modeling and characterization of our scalable microwave nanoprobe, which is a micro-fabricated cantilever-based scanning microwave probe with separated excitation and sensing electrodes. Using finite-element analysis, the tip-sample interaction is modeled as small impedance changes between the tip electrode and the ground at our working frequencies near 1GHz. The equivalent lumped elements of the cantilever can be determined by transmission line simulation of the matching network, which routes the cantilever signals to 50 Ohm feed lines. In the microwave electronics, the background common-mode signal is cancelled before the amplifier stage so that high sensitivity (below 1 atto-Farad capacitance changes) is obtained. Experimental characterization of the microwave probes was performed on ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or topographical signals can be realized using different reflection modes of the probe.

Atomic-force-microscope-compatible near-field scanning microwave microscope with separated excitation and sensing probes

We present the design and experimental results of a near-field scanning microwave microscope working at a frequency of 1 GHz. Our microscope is unique in that the sensing probe is separated from the excitation electrode to significantly suppress the common-mode signal. Coplanar waveguides were patterned onto a silicon nitride cantilever interchangeable with atomic force microscope tips, which are robust for high speed scanning. In the contact mode that we are currently using, the numerical analysis shows that contrast comes from both the variation in local dielectric properties and the sample topography. Our microscope demonstrates the ability to achieve high resolution microwave images on buried structures, as well as nanoparticles, nanowires, and biological samples.