Academic Appointments

2016-17 Courses

Stanford Advisees

All Publications

  • Dual-Gate Modulation of Carrier Density and Disorder in an Oxide Two-Dimensional Electron System NANO LETTERS Chen, Z., Yuan, H., Xie, Y., Lu, D., Inoue, H., Hikita, Y., Bell, C., Hwang, H. Y. 2016; 16 (10): 6130-6136


    Carrier density and disorder are two crucial parameters that control the properties of correlated two-dimensional electron systems. In order to disentangle their individual contributions to quantum phenomena, independent tuning of these two parameters is required. Here, by utilizing a hybrid liquid/solid electric dual-gate geometry acting on the conducting LaAlO3/SrTiO3 heterointerface, we obtain an additional degree of freedom to strongly modify the electron confinement profile and thus the strength of interfacial scattering, independent from the carrier density. A dual-gate controlled nonlinear Hall effect is a direct manifestation of this profile, which can be quantitatively understood by a Poisson-Schrödinger sub-band model. In particular, the large nonlinear dielectric response of SrTiO3 enables a very wide range of tunable density and disorder, far beyond that for conventional semiconductors. Our study provides a broad framework for understanding various reported phenomena at the LaAlO3/SrTiO3 interface.

    View details for DOI 10.1021/acs.nanolett.6b02348

    View details for Web of Science ID 000385469800023

    View details for PubMedID 27605459

  • Synthesis of freestanding single-crystal perovskite films and heterostructures by etching of sacrificial water-soluble layers. Nature materials Lu, D., Baek, D. J., Hong, S. S., Kourkoutis, L. F., Hikita, Y., Hwang, H. Y. 2016


    The ability to create and manipulate materials in two-dimensional (2D) form has repeatedly had transformative impact on science and technology. In parallel with the exfoliation and stacking of intrinsically layered crystals, atomic-scale thin film growth of complex materials has enabled the creation of artificial 2D heterostructures with novel functionality and emergent phenomena, as seen in perovskite heterostructures. However, separation of these layers from the growth substrate has proved challenging, limiting the manipulation capabilities of these heterostructures with respect to exfoliated materials. Here we present a general method to create freestanding perovskite membranes. The key is the epitaxial growth of water-soluble Sr 3Al 2O 6 on perovskite substrates, followed by in situ growth of films and heterostructures. Millimetre-size single-crystalline membranes are produced by etching the Sr 3Al 2O 6 layer in water, providing the opportunity to transfer them to arbitrary substrates and integrate them with heterostructures of semiconductors and layered compounds.

    View details for DOI 10.1038/nmat4749

    View details for PubMedID 27618712

  • A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction SCIENCE Seitz, L. C., Dickens, C. F., Nishio, K., Hikita, Y., Montoya, J., Doyle, A., Kirk, C., Vojvodic, A., Hwang, H. Y., Norskov, J. K., Jaramillo, T. F. 2016; 353 (6303): 1011-1014


    Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrOx/SrIrO3) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO3 This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO3 or anatase IrO2 motifs. The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.

    View details for DOI 10.1126/science.aaf5050

    View details for Web of Science ID 000382558900034

    View details for PubMedID 27701108

  • Defect Control of Conventional and Anomalous Electron Transport at Complex Oxide Interfaces PHYSICAL REVIEW X Gunkel, F., Bell, C., Inoue, H., Kim, B., Swartz, A. G., Merz, T. A., Hikita, Y., Harashima, S., Sato, H. K., Minohara, M., Hoffmann-Eifert, S., Dittmann, R., Hwang, H. Y. 2016; 6 (3)
  • Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and Their Monolayer Limit. Nano letters Yuan, H., Liu, Z., Xu, G., Zhou, B., Wu, S., Dumcenco, D., Yan, K., Zhang, Y., Mo, S., Dudin, P., Kandyba, V., Yablonskikh, M., Barinov, A., Shen, Z., Zhang, S., Huang, Y., Xu, X., Hussain, Z., Hwang, H. Y., Cui, Y., Chen, Y. 2016; 16 (8): 4738-4745


    Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, as well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.

    View details for DOI 10.1021/acs.nanolett.5b05107

    View details for PubMedID 27357620

  • Anisotropic Transport at the LaAlO3/SrTiO3 Interface Explained by Microscopic Imaging of Channel-Flow over SrTiO3 Domains ACS APPLIED MATERIALS & INTERFACES Frenkel, Y., Haham, N., Shperber, Y., Bell, C., Xie, Y., Chen, Z., Hikita, Y., Hwang, H. Y., Kalisky, B. 2016; 8 (19): 12514-12519


    Oxide interfaces, including the LaAlO3/SrTiO3 interface, have been a subject of intense interest for over a decade due to their rich physics and potential as low-dimensional nanoelectronic systems. The field has reached the stage where efforts are invested in developing devices. It is critical now to understand the functionalities and limitations of such devices. Recent scanning probe measurements of the LaAlO3/SrTiO3 interface have revealed locally enhanced current flow and accumulation of charge along channels related to SrTiO3 structural domains. These observations raised a key question regarding the role these modulations play in the macroscopic properties of devices. Here we show that the microscopic picture, mapped by scanning superconducting quantum interference device, accounts for a substantial part of the macroscopically measured transport anisotropy. We compared local flux data with transport values, measured simultaneously, over various SrTiO3 domain configurations. We show a clear relation between maps of local current density over specific domain configurations and the measured anisotropy for the same device. The domains divert the direction of current flow, resulting in a direction-dependent resistance. We also show that the modulation can be significant and that in some cases up to 95% of the current is modulated over the channels. The orientation and distribution of the SrTiO3 structural domains change between different cooldowns of the same device or when electric fields are applied, affecting the device behavior. Our results, highlight the importance of substrate physics, and in particular, the role of structural domains, in controlling electronic properties of LaAlO3/SrTiO3 devices. Furthermore, these results point to new research directions, exploiting the STO domains' ability to divert or even carry current.

    View details for DOI 10.1021/acsami.6b01655

    View details for Web of Science ID 000376330800069

    View details for PubMedID 27111600

  • Band Edge Engineering of Oxide Photoanodes for Photoelectrochemical Water Splitting: Integration of Subsurface Dipoles with Atomic-Scale Control ADVANCED ENERGY MATERIALS Hikita, Y., Nishio, K., Seitz, L. C., Chakthranont, P., Tachikawa, T., Jaramillo, T. F., Hwang, H. Y. 2016; 6 (7)
  • High Responsivity Phototransistors Based on Few-Layer ReS2 for Weak Signal Detection ADVANCED FUNCTIONAL MATERIALS Liu, E., Long, M., Zeng, J., Luo, W., Wang, Y., Pan, Y., Zhou, W., Wang, B., Hu, W., Ni, Z., You, Y., Zhang, X., Qin, S., Shi, Y., Watanabe, K., Taniguchi, T., Yuan, H., Hwang, H. Y., Cui, Y., Miao, F., Xing, D. 2016; 26 (12): 1938-1944
  • Thermodynamic guiding principles in selective synthesis of strontium iridate Ruddlesden-Popper epitaxial films APL MATERIALS Nishio, K., Hwang, H. Y., Hikita, Y. 2016; 4 (3)

    View details for DOI 10.1063/1.4943519

    View details for Web of Science ID 000373685100015

  • Origin of the Magnetoresistance in Oxide Tunnel Junctions Determined through Electric Polarization Control of the Interface PHYSICAL REVIEW X Inoue, H., Swartz, A. G., Harmon, N. J., Tachikawa, T., Hikita, Y., Flatte, M. E., Hwang, H. Y. 2015; 5 (4)
  • Inelastic x-ray scattering in heterostructures: electronic excitations in LaAlO3/SrTiO3 JOURNAL OF PHYSICS-CONDENSED MATTER Ruotsalainen, K. O., Sahle, C. J., Ritschel, T., Geck, J., Hosoda, M., Bell, C., Hikita, Y., Hwang, H. Y., Fister, T. T., Gordon, R. A., Hamalainen, K., Hakala, M., Huotari, S. 2015; 27 (33)


    We present an investigation of the valence-electron excitation spectra including the collective plasmon modes of SrTiO3, LaAlO3 and their heterostructures with non-resonant inelastic x-ray scattering. We analyse the spectra using calculations based on first principles and atomic multiplet models. We demonstrate the feasibility of performing valence IXS experiments in a total reflection geometry. Surprisingly, we find that the plasmon, interband and semicore excitations in multilayers are well described as a superposition of bulk-compound spectra even in a superstructure composing of layers of only one atomic layer thickness.

    View details for DOI 10.1088/0953-8984/27/33/335501

    View details for Web of Science ID 000359080100005

    View details for PubMedID 26221981

  • Polarization-sensitive broadband photodetector using a black phosphorus vertical p-n junction NATURE NANOTECHNOLOGY Yuan, H., Liu, X., Afshinmanesh, F., Li, W., Xu, G., Sun, J., Lian, B., Curto, A. G., Ye, G., Hikita, Y., Shen, Z., Zhang, S., Chen, X., Brongersma, M., Hwang, H. Y., Cui, Y. 2015; 10 (8): 707-713


    The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodetector using a layered black phosphorus transistor that is polarization-sensitive over a bandwidth from ∼400 nm to 3,750 nm. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material. In this transistor geometry, a perpendicular built-in electric field induced by gating can spatially separate the photogenerated electrons and holes in the channel, effectively reducing their recombination rate and thus enhancing the performance for linear dichroism photodetection. The use of anisotropic layered black phosphorus in polarization-sensitive photodetection might provide new functionalities in novel optical and optoelectronic device applications.

    View details for DOI 10.1038/NNANO.2015.112

    View details for Web of Science ID 000359754500016

    View details for PubMedID 26030655

  • Direct Imaging of Nanoscale Conductance Evolution in Ion-Gel-Gated Oxide Transistors NANO LETTERS Ren, Y., Yuan, H., Wu, X., Chen, Z., Iwasa, Y., Cui, Y., Hwang, H. Y., Lai, K. 2015; 15 (7): 4730-4736


    Electrostatic modification of functional materials by electrolytic gating has demonstrated a remarkably wide range of density modulation, a condition crucial for developing novel electronic phases in systems ranging from complex oxides to layered chalcogenides. Yet little is known microscopically when carriers are modulated in electrolyte-gated electric double-layer transistors (EDLTs) due to the technical challenge of imaging the buried electrolyte-semiconductor interface. Here, we demonstrate the real-space mapping of the channel conductance in ZnO EDLTs using a cryogenic microwave impedance microscope. A spin-coated ionic gel layer with typical thicknesses below 50 nm allows us to perform high resolution (on the order of 100 nm) subsurface imaging, while maintaining the capability of inducing the metal-insulator transition under a gate bias. The microwave images vividly show the spatial evolution of channel conductance and its local fluctuations through the transition as well as the uneven conductance distribution established by a large source-drain bias. The unique combination of ultrathin ion-gel gating and microwave imaging offers a new opportunity to study the local transport and mesoscopic electronic properties in EDLTs.

    View details for DOI 10.1021/acs.nanolett.5b01631

    View details for Web of Science ID 000357964100073

    View details for PubMedID 26061780

  • Pressure induced metallization with absence of structural transition in layered molybdenum diselenide NATURE COMMUNICATIONS Zhao, Z., Zhang, H., Yuan, H., Wang, S., Lin, Y., Zeng, Q., Xu, G., Liu, Z., Solanki, G. K., Patel, K. D., Cui, Y., Hwang, H. Y., Mao, W. L. 2015; 6

    View details for DOI 10.1038/ncomms8312

    View details for Web of Science ID 000357171600016

  • Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors NATURE COMMUNICATIONS Liu, E., Fu, Y., Wang, Y., Feng, Y., Liu, H., Wan, X., Zhou, W., Wang, B., Shao, L., Ho, C., Huang, Y., Cao, Z., Wang, L., Li, A., Zeng, J., Song, F., Wang, X., Shi, Y., Yuan, H., Hwang, H. Y., Cui, Y., Miao, F., Xing, D. 2015; 6

    View details for DOI 10.1038/ncomms7991

    View details for Web of Science ID 000355528800001

  • Controlling band alignments by artificial interface dipoles at perovskite heterointerfaces NATURE COMMUNICATIONS Yajima, T., Hikita, Y., Minohara, M., Bell, C., Mundy, J. A., Kourkoutis, L. F., Muller, D. A., Kumigashira, H., Oshima, M., Hwang, H. Y. 2015; 6


    The concept 'the interface is the device' is embodied in a wide variety of interfacial electronic phenomena and associated applications in oxide materials, ranging from catalysts and clean energy systems to emerging multifunctional devices. Many device properties are defined by the band alignment, which is often influenced by interface dipoles. On the other hand, the ability to purposefully create and control interface dipoles is a relatively unexplored degree of freedom for perovskite oxides, which should be particularly effective for such ionic materials. Here we demonstrate tuning the band alignment in perovskite metal-semiconductor heterojunctions over a broad range of 1.7 eV. This is achieved by the insertion of positive or negative charges at the interface, and the resultant dipole formed by the induced screening charge. This approach can be broadly used in applications where decoupling the band alignment from the constituent work functions and electron affinities can enhance device functionality.

    View details for DOI 10.1038/ncomms7759

    View details for Web of Science ID 000353702300016

    View details for PubMedID 25849738

  • Room temperature optical anisotropy of a LaMnO3 thin-film induced by ultra-short pulse laser APPLIED PHYSICS LETTERS Munkhbaatar, P., Marton, Z., Tsermaa, B., Choi, W. S., Seo, S. S., Kim, J. S., Nakagawa, N., Hwang, H. Y., Lee, H. N., Myung-Whun, K. 2015; 106 (9)

    View details for DOI 10.1063/1.4914094

    View details for Web of Science ID 000351069900045

  • Enhanced Electrical Transparency by Ultrathin LaAlO3 Insertion at Oxide Metal/Semiconductor Heterointerfaces NANO LETTERS Yajima, T., Minohara, M., Bell, C., Kumigashira, H., Oshima, M., Hwang, H. Y., Hikita, Y. 2015; 15 (3): 1622-1626


    We demonstrate that the electrical conductivity of metal/semiconductor oxide heterojunctions can be increased over 7 orders of magnitude by inserting an ultrathin layer of LaAlO3. This counterintuitive result, that an interfacial barrier can be driven transparent by inserting a wide-gap insulator, arises from the large internal electric field between the two polar LaAlO3 surfaces. This field modifies the effective band offset in the device, highlighting the ability to design the electrostatic boundary conditions with atomic precision.

    View details for DOI 10.1021/nl504169m

    View details for Web of Science ID 000351188000027

    View details for PubMedID 25654211

  • Electrically Tunable Coherent Optical Absorption in Graphene with Ion Gel NANO LETTERS Thareja, V., Kang, J., Yuan, H., Milaninia, K. M., Hwang, H. Y., Cui, Y., Kik, P. G., Brongersma, M. L. 2015; 15 (3): 1570-1576


    We demonstrate electrical control over coherent optical absorption in a graphene-based Salisbury screen consisting of a single layer of graphene placed in close proximity to a gold back reflector. The screen was designed to enhance light absorption at a target wavelength of 3.2 μm by using a 600 nm-thick, nonabsorbing silica spacer layer. An ionic gel layer placed on top of the screen was used to electrically gate the charge density in the graphene layer. Spectroscopic reflectance measurements were performed in situ as a function of gate bias. The changes in the reflectance spectra were analyzed using a Fresnel based transfer matrix model in which graphene was treated as an infinitesimally thin sheet with a conductivity given by the Kubo formula. The analysis reveals that a careful choice of the ionic gel layer thickness can lead to optical absorption enhancements of up to 5.5 times for the Salisbury screen compared to a suspended sheet of graphene. In addition to these absorption enhancements, we demonstrate very large electrically induced changes in the optical absorption of graphene of ∼3.3% per volt, the highest attained so far in a device that features an atomically thick active layer. This is attributable in part to the more effective gating achieved with the ion gel over the conventional dielectric back gates and partially by achieving a desirable coherent absorption effect linked to the presence of the thin ion gel that boosts the absorption by 40%.

    View details for DOI 10.1021/nl503431d

    View details for Web of Science ID 000351188000019

  • Pressure induced metallization with absence of structural transition in layered molybdenum diselenide. Nature communications Zhao, Z., Zhang, H., Yuan, H., Wang, S., Lin, Y., Zeng, Q., Xu, G., Liu, Z., Solanki, G. K., Patel, K. D., Cui, Y., Hwang, H. Y., Mao, W. L. 2015; 6: 7312-?


    Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ∼60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.

    View details for DOI 10.1038/ncomms8312

    View details for PubMedID 26088416

  • Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors. Nature communications Liu, E., Fu, Y., Wang, Y., Feng, Y., Liu, H., Wan, X., Zhou, W., Wang, B., Shao, L., Ho, C., Huang, Y., Cao, Z., Wang, L., Li, A., Zeng, J., Song, F., Wang, X., Shi, Y., Yuan, H., Hwang, H. Y., Cui, Y., Miao, F., Xing, D. 2015; 6: 6991-?


    Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼10(7)) and low subthreshold swings (100 mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications.

    View details for DOI 10.1038/ncomms7991

    View details for PubMedID 25947630

  • Large-Scale Production of Graphene Nanoribbons from Electrospun Polymers JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Liu, N., Kim, K., Hsu, P., Sokolov, A. N., Yap, F. L., Yuan, H., Xie, Y., Yan, H., Cui, Y., Hwang, H. Y., Bao, Z. 2014; 136 (49): 17284-17291


    Graphene nanoribbons (GNRs) are promising building blocks for high-performance electronics due to their high electron mobility and dimensionality-induced bandgap. Despite many past efforts, direct synthesis of GNRs with controlled dimensions and scalability remains challenging. Here we report the scalable synthesis of GNRs using electrospun polymer nanofiber templates. Palladium-incorporated poly(4-vinylphenol) nanofibers were prepared by electrospinning with controlled diameter and orientation. Highly graphitized GNRs as narrow as 10 nm were then synthesized from these templates by chemical vapor deposition. A transport gap can be observed in 30 nm-wide GNRs, enabling them to function as field-effect transistors at room temperature. Our results represent the first success on the scalable synthesis of highly graphitized GNRs from polymer templates. Furthermore, the generality of this method allows various polymers to be explored, which will lead to understanding of growth mechanism and rational control over crystallinity, feature size and bandgap to enable a new pathway for graphene electronics.

    View details for DOI 10.1021/ja509871n

    View details for Web of Science ID 000346544200045

    View details for PubMedID 25407608

  • Quantification and impact of nonparabolicity of the conduction band of indium tin oxide on its plasmonic properties APPLIED PHYSICS LETTERS Liu, X., Park, J., Kang, J., Yuan, H., Cui, Y., Hwang, H. Y., Brongersma, M. L. 2014; 105 (18)

    View details for DOI 10.1063/1.4900936

    View details for Web of Science ID 000345000000017

  • Quantum longitudinal and Hall transport at the LaAlO3/SrTiO3 interface at low electron densities SOLID STATE COMMUNICATIONS Xie, Y., Bell, C., Kim, M., Inoue, H., Hikita, Y., Hwang, H. Y. 2014; 197: 25-29
  • Atomically Engineered Metal Insulator Transition at the TiO2/LaAlO3 Heterointerface NANO LETTERS Minohara, M., Tachikawa, T., Nakanishi, Y., Hikita, Y., Kourkoutis, L. F., Lee, J., Kao, C., Yoshita, M., Akiyama, H., Bell, C., Hwang, H. Y. 2014; 14 (11): 6743-6746


    We demonstrate that the atomic boundary conditions of simple binary oxides can be used to impart dramatic changes of state. By changing the substrate surface termination of LaAlO3 (001) from AlO2 to LaO, the room-temperature sheet conductance of anatase TiO2 films are increased by over 3 orders of magnitude, transforming the intrinsic insulating state to a high mobility metallic state, while maintaining excellent optical transparency.

    View details for DOI 10.1021/nl5039192

    View details for Web of Science ID 000345723800113

    View details for PubMedID 25343440

  • Generation and electric control of spin-valley-coupled circular photogalvanic current in WSe2 NATURE NANOTECHNOLOGY Yuan, H., Wang, X., Lian, B., Zhang, H., Fang, X., Shen, B., Xu, G., Xu, Y., Zhang, S., Hwang, H. Y., Cui, Y. 2014; 9 (10): 851-857
  • Spin-dependent transport across Co/LaAlO3/SrTiO3 heterojunctions APPLIED PHYSICS LETTERS Swartz, A. G., Harashima, S., Xie, Y., Lu, D., Kim, B., Bell, C., Hikita, Y., Hwang, H. Y. 2014; 105 (3)

    View details for DOI 10.1063/1.4891174

    View details for Web of Science ID 000341152300049

  • Spatial density profile of electrons near the LaAlO3/SrTiO3 heterointerface revealed by time-resolved photoluminescence spectroscopy APPLIED PHYSICS LETTERS Yamada, Y., Sato, H. K., Hikita, Y., Hwang, H. Y., Kanemitsu, Y. 2014; 104 (15)

    View details for DOI 10.1063/1.4872171

    View details for Web of Science ID 000335145200022

  • Visualizing the interfacial evolution from charge compensation to metallic screening across the manganite metal-insulator transition NATURE COMMUNICATIONS Mundy, J. A., Hikita, Y., Hidaka, T., Yajima, T., Higuchi, T., Hwang, H. Y., Muller, D. A., Kourkoutis, L. F. 2014; 5


    Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in devices. The nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Here we directly quantify with atomic-resolution the charge distribution for manganite-titanate interfaces traversing the metal-insulator transition. By measuring the concentration and valence of the cations, we find an intrinsic interfacial electronic reconstruction in the insulating films. The total charge observed for the insulating manganite films quantitatively agrees with that needed to cancel the polar catastrophe. As the manganite becomes metallic with increased hole doping, the total charge build-up and its spatial range drop substantially. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases.

    View details for DOI 10.1038/ncomms4464

    View details for Web of Science ID 000334300400047

    View details for PubMedID 24632721

  • Tunable coupling of two-dimensional superconductors in bilayer SrTiO3 heterostructures PHYSICAL REVIEW B Inoue, H., Kim, M., Bell, C., Hikita, Y., Raghu, S., Hwang, H. Y. 2013; 88 (24)
  • Locally enhanced conductivity due to the tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces. Nature materials Kalisky, B., Spanton, E. M., Noad, H., Kirtley, J. R., Nowack, K. C., Bell, C., Sato, H. K., Hosoda, M., Xie, Y., Hikita, Y., Woltmann, C., Pfanzelt, G., Jany, R., Richter, C., Hwang, H. Y., Mannhart, J., Moler, K. A. 2013; 12 (12): 1091-1095


    The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO3 and TiO2-terminated SrTiO3 (STO; refs , ). Transport and other measurements in this system show a plethora of diverse physical phenomena. To better understand the interface conductivity, we used scanning superconducting quantum interference device microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed on thermal cycling above the STO cubic-to-tetragonal structural transition temperature, implying that the local conductivity is strongly modified by the STO tetragonal domain structure. The interplay between substrate domains and the interface provides an additional mechanism for understanding and controlling the behaviour of heterostructures.

    View details for DOI 10.1038/nmat3753

    View details for PubMedID 24013791

  • Landau Level Spectroscopy of Dirac Electrons in a Polar Semiconductor with Giant Rashba Spin Splitting PHYSICAL REVIEW LETTERS Bordacs, S., Checkelsky, J. G., Murakawa, H., Hwang, H. Y., Tokura, Y. 2013; 111 (16)


    Optical excitations of BiTeI with large Rashba spin splitting have been studied in an external magnetic field (B) applied parallel to the polar axis. A sequence of transitions between the Landau levels (LLs), whose energies are in proportion to √B were observed, being characteristic of massless Dirac electrons. The large separation energy between the LLs makes it possible to detect the strongest cyclotron resonance even at room temperature in moderate fields. Unlike in 2D Dirac systems, the magnetic field induced rearrangement of the conductivity spectrum is directly observed.

    View details for DOI 10.1103/PhysRevLett.111.166403

    View details for Web of Science ID 000326148000013

    View details for PubMedID 24182286

  • Controlled Growth of High-Quality Monolayer WS2 Layers on Sapphire and Imaging Its Grain Boundary ACS NANO Zhang, Y., Zhang, Y., Ji, Q., Ju, J., Yuan, H., Shi, J., Gao, T., Ma, D., Liu, M., Chen, Y., Song, X., Hwang, H. Y., Cui, Y., Liu, Z. 2013; 7 (10): 8963-8971


    Atomically thin tungsten disulfide (WS2), a structural analogue to MoS2, has attracted great interest due to its indirect-to-direct band-gap tunability, giant spin splitting, and valley-related physics. However, the batch production of layered WS2 is underdeveloped (as compared with that of MoS2) for exploring these fundamental issues and developing its applications. Here, using a low-pressure chemical vapor deposition method, we demonstrate that high-crystalline mono- and few-layer WS2 flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm(2). Intriguingly, we show that, with adding minor H2 carrier gas, the shape of monolayer WS2 flakes can be tailored from jagged to straight edge triangles and still single crystalline. Meanwhile, some intersecting triangle shape flakes are concomitantly evolved from more than one nucleus to show a polycrystalline nature. It is interesting to see that, only through a mild sample oxidation process, the grain boundaries are easily recognizable by scanning electron microscopy due to its altered contrasts. Hereby, controlling the initial nucleation state is crucial for synthesizing large-scale single-crystalline flakes. We believe that this work would benefit the controlled growth of high-quality transition metal dichalcogenide, as well as in their future applications in nanoelectronics, optoelectronics, and solar energy conversions.

    View details for DOI 10.1021/nn403454e

    View details for Web of Science ID 000326209100068

    View details for PubMedID 24047054

  • Enhancing Electron Mobility at the LaAlO3/SrTiO3 Interface by Surface Control ADVANCED MATERIALS Xie, Y., Bell, C., Hikita, Y., Harashima, S., Hwang, H. Y. 2013; 25 (34): 4735-4738


    Mobility of electrons confined at the LaAlO3 /SrTiO3 interface is significantly enhanced by surface control using surface charges and adsorbates, reaching a low temperature value more than 20 000 cm(2) V(-1) s(-1) . A uniform trend that mobility increases with decreasing sheet carrier density is observed.

    View details for DOI 10.1002/adma.201301798

    View details for Web of Science ID 000327692100013

  • Transistor operation and mobility enhancement in top-gated LaAlO3/SrTiO3 heterostructures APPLIED PHYSICS LETTERS Hosoda, M., Hikita, Y., Hwang, H. Y., Bell, C. 2013; 103 (10)

    View details for DOI 10.1063/1.4820449

    View details for Web of Science ID 000324389700061

  • Measurement of the Femtosecond Optical Absorption of LaAlO3/SrTiO3 Heterostructures: Evidence for an Extremely Slow Electron Relaxation at the Interface PHYSICAL REVIEW LETTERS Yamada, Y., Sato, H. K., Hikita, Y., Hwang, H. Y., Kanemitsu, Y. 2013; 111 (4)


    The photocarrier relaxation dynamics of an n-type LaAlO_{3}/SrTiO_{3} heterointerface is investigated using femtosecond transient absorption (TA) spectroscopy at low temperatures. In both LaAlO_{3}/SrTiO_{3} heterostructures and electron-doped SrTiO_{3} bulk crystals, the TA spectrum shows a Drude-like free carrier absorption immediately after excitation. In addition, a broad absorption band gradually appears within 40 ps, which corresponds to the energy relaxation of photoexcited free electrons into self-trapped polaron states. We reveal that the polaron formation time is enhanced considerably at the LaAlO_{3}/SrTiO_{3} heterointerface as compared to bulk crystals. Further, we discuss the interface effects on the electron relaxation dynamics in conjunction with the splitting of the t_{2g} subbands due to the interface potential.

    View details for DOI 10.1103/PhysRevLett.111.047403

    View details for Web of Science ID 000322218000013

    View details for PubMedID 23931405

  • Compositional and gate tuning of the interfacial conductivity in LaAlO3/LaTiO3/SrTiO3 heterostructures APPLIED PHYSICS LETTERS Hosoda, M., Bell, C., Hikita, Y., Hwang, H. Y. 2013; 102 (9)

    View details for DOI 10.1063/1.4794410

    View details for Web of Science ID 000316085200013

  • Hot electron transport in a strongly correlated transition-metal oxide SCIENTIFIC REPORTS Rana, K. G., Yajima, T., Parui, S., Kemper, A. F., Devereaux, T. P., Hikita, Y., Hwang, H. Y., Banerjee, T. 2013; 3


    Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant for oxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La?.?Sr?.?MnO? (LSMO)- Nb-doped SrTiO? (Nb:STO) interface and find the characteristic hot-electron attenuation length in LSMO to be 1.48 ± 0.10 unit cells (u.c.) at -1.9?V, increasing to 2.02 ± 0.16?u.c. at -1.3?V at room temperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron and polaron scattering. Direct visualization of the local electron transport shows different transmission at the terraces and at the step-edges.

    View details for DOI 10.1038/srep01274

    View details for Web of Science ID 000314864800002

    View details for PubMedID 23429420

  • Gate-tuned superfluid density at the superconducting LaAlO3/SrTiO3 interface PHYSICAL REVIEW B Bert, J. A., Nowack, K. C., Kalisky, B., Noad, H., Kirtley, J. R., Bell, C., Sato, H. K., Hosoda, M., Hikita, Y., Hwang, H. Y., Moler, K. A. 2012; 86 (6)
  • Scanning Probe Manipulation of Magnetism at the LaAlO3/SrTiO3 Heterointerface NANO LETTERS Kalisky, B., Bert, J. A., Bell, C., Xie, Y., Sato, H. K., Hosoda, M., Hikita, Y., Hwang, H. Y., Moler, K. A. 2012; 12 (8): 4055-4059


    Manipulation of magnetism is a longstanding goal of research in exotic materials. In this work, we demonstrate that the small ferromagnetic patches in LaAlO(3)/SrTiO(3) heterostructures can be dramatically changed by in situ contact of a scanning probe. Our results provide a platform for manipulation of small magnets through either a strong magneto-elastic coupling or sensitivity to surface modification. The ability to locally control magnetism is particularly interesting due to the presence of superconductivity with strong spin-orbit coupling in LaAlO(3)/SrTiO(3).

    View details for DOI 10.1021/nl301451e

    View details for Web of Science ID 000307211000030

    View details for PubMedID 22769056

  • Critical thickness for ferromagnetism in LaAlO3/SrTiO3 heterostructures NATURE COMMUNICATIONS Kalisky, B., Bert, J. A., Klopfer, B. B., Bell, C., Sato, H. K., Hosoda, M., Hikita, Y., Hwang, H. Y., Moler, K. A. 2012; 3


    In LaAlO(3)/SrTiO(3) heterointerfaces, charge carriers migrate from the LaAlO(3) to the interface in an electronic reconstruction. Magnetism has been observed in LaAlO(3)/SrTiO(3), but its relationship to the interface conductivity is unknown. Here we show that reconstruction is necessary, but not sufficient, for the formation of magnetism. Using scanning superconducting quantum interference device microscopy we find that magnetism appears only above a critical LaAlO(3) thickness, similar to the conductivity. We observe no change in ferromagnetism with gate voltage, and detect ferromagnetism in a non-conducting p-type sample. These observations indicate that the carriers at the interface do not need to be itinerant to generate magnetism. The ferromagnetism appears in isolated patches whose density varies greatly between samples. This inhomogeneity strongly suggests that disorder or local strain generates magnetism in a population of the interface carriers.

    View details for DOI 10.1038/ncomms1931

    View details for Web of Science ID 000306099900049

    View details for PubMedID 22735450

  • Control of electronic conduction at an oxide heterointerface using surface polar adsorbates NATURE COMMUNICATIONS Xie, Y., Hikita, Y., Bell, C., Hwang, H. Y. 2011; 2


    The interface between LaAlO(3) and SrTiO(3) possesses a range of intriguing properties, notably a proposed connection between the surface state of the LaAlO(3) and the conductivity buried in the SrTiO(3). Here we study the effect of the surface adsorption of a variety of common laboratory solvents on the conductivity at the interface between LaAlO(3) and SrTiO(3). We show that the application of chemicals such as acetone, ethanol, and water can induce a large change in the conductivity, and, in particular, an insulator to metal transition around the critical LaAlO(3) thickness. This phenomenon is observed only for polar solvents. These data provide experimental evidence for a general polarization-facilitated electronic transfer mechanism.

    View details for DOI 10.1038/ncomms1501

    View details for Web of Science ID 000296787300007

    View details for PubMedID 21988910

  • Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface NATURE PHYSICS Bert, J. A., Kalisky, B., Bell, C., Kim, M., Hikita, Y., Hwang, H. Y., Moler, K. A. 2011; 7 (10): 767-771

    View details for DOI 10.1038/nphys2079

    View details for Web of Science ID 000295584200013

  • Reentrant insulating state in ultrathin manganite films APPLIED PHYSICS LETTERS Kim, B., Kwon, D., Yajima, T., Bell, C., Hikita, Y., Kim, B. G., Hwang, H. Y. 2011; 99 (9)

    View details for DOI 10.1063/1.3628659

    View details for Web of Science ID 000294489300048

  • Structural Comparison of n-Type and p-Type LaAlO3/SrTiO3 Interfaces PHYSICAL REVIEW LETTERS Yamamoto, R., Bell, C., Hikita, Y., Hwang, H. Y., Nakamura, H., Kimura, T., Wakabayashi, Y. 2011; 107 (3)


    Using a surface x-ray diffraction technique, we investigated the atomic structure of two types of interfaces between LaAlO3 and SrTiO3, that is, p-type (SrO/AlO2) and n-type (TiO2/LaO) interfaces. Our results demonstrate that the SrTiO3 in the sample with the n-type interface has a large polarized region, while that with the p-type interface has a limited polarized region. In addition, atomic intermixing was observed to extend deeper into the SrTiO3 substrate at the n-type interface compared to the p type. These differences result in distinct degrees of band bending, which likely contributes to the striking contrast in electrical conductivity between the two types of interfaces.

    View details for DOI 10.1103/PhysRevLett.107.036104

    View details for Web of Science ID 000292764600006

    View details for PubMedID 21838380

  • Tuning the Electron Gas at an Oxide Heterointerface via Free Surface Charges ADVANCED MATERIALS Xie, Y., Bell, C., Hikita, Y., Hwang, H. Y. 2011; 23 (15): 1744-?

    View details for DOI 10.1002/adma.201004673

    View details for Web of Science ID 000289531800008

    View details for PubMedID 21400617

  • A heteroepitaxial perovskite metal-base transistor NATURE MATERIALS Yajima, T., Hikita, Y., Hwang, H. Y. 2011; 10 (3): 198-201


    'More than Moore' captures a concept for overcoming limitations in silicon electronics by incorporating new functionalities in the constituent materials. Perovskite oxides are candidates because of their vast array of physical properties in a common structure. They also enable new electronic devices based on strongly-correlated electrons. The field effect transistor and its derivatives have been the principal oxide devices investigated thus far, but another option is available in a different geometry: if the current is perpendicular to the interface, the strong internal electric fields generated at back-to-back heterojunctions can be used for oxide electronics, analogous to bipolar transistors. Here we demonstrate a perovskite heteroepitaxial metal-base transistor operating at room temperature, enabled by interface dipole engineering. Analysis of many devices quantifies the evolution from hot-electron to permeable-base behaviour. This device provides a platform for incorporating the exotic ground states of perovskite oxides, as well as novel electronic phases at their interfaces.

    View details for DOI 10.1038/NMAT2946

    View details for Web of Science ID 000287517400019

    View details for PubMedID 21258354