Adam W. Tsen

Tailoring Electrical Transport Across Grain Boundaries in Polycrystalline Graphene

Going Up Against the Grain Boundaries Exfoliated graphene sheets are single crystals that exhibit excellent electronic properties, but their fabrication is too slow for large-scale device fabrication. Growth methods such as chemical vapor deposition are faster, but create polycrystalline graphene sheets that contain grain boundaries that can scatter charge carriers and decrease performance. Tsen et al. (p. 1143) found that the presence of overlapping domains within polycrystalline graphene samples could increase conductivity of samples by an order of magnitude, allowing them to rival exfoliated samples. Overlap between crystallites in vapor-grown graphene improves electronic conductivity. Graphene produced by chemical vapor deposition (CVD) is polycrystalline, and scattering of charge carriers at grain boundaries (GBs) could degrade its performance relative to exfoliated, single-crystal graphene. However, the electrical properties of GBs have so far been addressed indirectly without simultaneous knowledge of their locations and structures. We present electrical measurements on individual GBs in CVD graphene first imaged by transmission electron microscopy. Unexpectedly, the electrical conductance improves by one order of magnitude for GBs with better interdomain connectivity. Our study suggests that polycrystalline graphene with good stitching may allow for uniformly high electrical performance rivaling that of exfoliated samples, which we demonstrate using optimized growth conditions and device geometry.

Strain solitons and topological defects in bilayer graphene

Bilayer graphene has been a subject of intense study in recent years. The interlayer registry between the layers can have dramatic effects on the electronic properties: for example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene [Oostinga JB, et al. (2007) Nature Materials 7:151–157]. This band gap is intimately tied to a structural spontaneous symmetry breaking in bilayer graphene, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in multiple stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries [Vaezi A, et al. (2013) arXiv:1301.1690; Zhang F, et al. (2013) arXiv:1301.4205], but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6–11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in situ heating above 1,000 °C. The abundance of these structures across a variety of samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene.

Photocurrent Imaging of p-n Junctions in Ambipolar Carbon Nanotube Transistors

We use scanning photocurrent microscopy (SPCM) to investigate the properties of internal p-n junctions in ambipolar carbon nanotube (CNT) transistors. Our SPCM images show strong signals near metal contacts whose polarity and positions change depending on the gate bias. SPCM images analyzed in conjunction with the overall conductance also indicate the existence and gate-dependent evolution of internal p-n junctions near contacts in the n-type operation regime. To determine the p-n junction position and the depletion width with a nanometer scale resolution, a Gaussian fit was used. We also measure the electric potential profile of partially suspended CNT devices at different gate biases, which shows that induced local fields can be imaged using the SPCM technique. Our experiment clearly demonstrates that SPCM is a valuable tool for imaging and optimizing electrical and optoelectronic properties of CNT based devices.

Nature of the quantum metal in a two-dimensional crystalline superconductor

Owing to electron localization, two-dimensional materials are not expected to be metallic at low temperatures, but a field-induced quantum metal phase emerges in NbSe2, whose behaviour is consistent with the Bose-metal model.

Structure and control of charge density waves in two-dimensional 1T-TaS2

Significance The ability to electrically control collective electron states is a central goal of materials research and may allow for the development of novel devices. 1T-TaS2 is an ideal candidate for such devices due to the existence of various charge ordered states in its phase diagram. Although various techniques have been demonstrated to manipulate charge order in 1T-TaS2, a fundamental understanding of the effects is still lacking, and the methods used are incompatible with device fabrication. By using both high-resolution transmission electron microscopy and electronic transport to investigate atomically thin 1T-TaS2 samples, we clarify the microscopic nature of the charge ordered phases in the 2D limit and further control them by all-electrical means. The layered transition metal dichalcogenides host a rich collection of charge density wave phases in which both the conduction electrons and the atomic structure display translational symmetry breaking. Manipulating these complex states by purely electronic methods has been a long-sought scientific and technological goal. Here, we show how this can be achieved in 1T-TaS2 in the 2D limit. We first demonstrate that the intrinsic properties of atomically thin flakes are preserved by encapsulation with hexagonal boron nitride in inert atmosphere. We use this facile assembly method together with transmission electron microscopy and transport measurements to probe the nature of the 2D state and show that its conductance is dominated by discommensurations. The discommensuration structure can be precisely tuned in few-layer samples by an in-plane electric current, allowing continuous electrical control over the discommensuration-melting transition in 2D.

Imaging the electrical conductance of individual carbon nanotubes with photothermal current microscopy

The one-dimensional structure of carbon nanotubes leads to a variety of remarkable optical and electrical properties that could be used to develop novel devices. Recently, the electrical conductance of nanotubes has been shown to decrease under optically induced heating by an amount proportional to the temperature change. Here, we show that this decrease is also proportional to the initial nanotube conductance, and make use of this effect to develop a new electrical characterization tool for nanotubes. By scanning the focal spot of a laser across the surface of a device through which current is simultaneously measured, we can construct spatially resolved conductance images of both single and arrayed nanotube transistors. We can also directly image the gate control of these devices. Our results establish photothermal current microscopy as an important addition to the existing suite of characterization techniques for carbon nanotubes and other linear nanostructures.

Atomic lattice disorder in charge-density-wave phases of exfoliated dichalcogenides (1T-TaS2)

Significance Low-dimensional materials, such as 1T-TaS2, permit unique phases that arise through electronic and structural reshaping known, respectively, as charge-density waves and periodic lattice distortions (PLDs). Determining the atomic structure of PLDs is critical toward understanding the origin of these charge-ordered phases and their effect on electronic properties. Here we reveal the microscopic nature of PLDs at cryogenic and room temperature in thin flakes of 1T-TaS2 using atomic resolution scanning transmission electron microscopy. Real-space characterization of the local PLD structure across the phase diagram will enable harnessing of emergent properties of thin transition-metal dichalcogenides. Charge-density waves (CDWs) and their concomitant periodic lattice distortions (PLDs) govern the electronic properties in several layered transition-metal dichalcogenides. In particular, 1T-TaS2 undergoes a metal-to-insulator phase transition as the PLD becomes commensurate with the crystal lattice. Here we directly image PLDs of the nearly commensurate (NC) and commensurate (C) phases in thin, exfoliated 1T-TaS2 using atomic resolution scanning transmission electron microscopy at room and cryogenic temperature. At low temperatures, we observe commensurate PLD superstructures, suggesting ordering of the CDWs both in- and out-of-plane. In addition, we discover stacking transitions in the atomic lattice that occur via one-bond-length shifts. Interestingly, the NC PLDs exist inside both the stacking domains and their boundaries. Transitions in stacking order are expected to create fractional shifts in the CDW between layers and may be another route to manipulate electronic phases in layered dichalcogenides.

Distinct surface and bulk charge density waves in ultrathin 1T-TaS 2

We employ low-frequency Raman spectroscopy to study the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition in 1T-TaS2 ultrathin flakes protected from oxidation. We identify new modes originating from C phase CDW phonons that are distinct from those seen in bulk 1T-TaS2. We attribute these to CDW modes from the surface layers. By monitoring individual modes with temperature, we find that surfaces undergo a separate, low-hysteresis NC-C phase transition that is decoupled from the transition in the bulk layers. This indicates the activation of a secondary phase nucleation process in the limit of weak interlayer interaction, which can be understood from energy considerations.

Photoelectrical imaging and characterization of point contacts in pentacene thin-film transistors

We report the spatially resolved electrical response of bottom-contact pentacene thin-film transistors to a scanning, focused laser. We find that pentacene films make point-like electrical contacts to the underlying gold electrodes and are able to image them with diffraction-limited resolution. We can further estimate the interfacial resistance associated with hole-injection at an individual point contact, and show that optical activation of one alone increases device current significantly.

Thickness and Stacking Sequence Determination of Exfoliated Dichalchogenides Using Scanning Transmission Electron Microscopy

Layered transition metal dichalcogenides (TMD) have attracted growing interest due to their promise for future technologies. As one approaches the 2D limit, the thickness and local topology can greatly influence the materials macroscopic properties [1]. To understand their potential for electronic applications it is therefore important to identify the dimension and atomic layer stacking of TMDs. The goal of this work is to extract the thickness and stacking sequence of TMDs directly by matching experimentally recorded HAADF images and convergent beam electron diffraction (CBED) patterns to multislice simulations. We demonstrate the accuracy to which thickness and stacking order can be determined in exfoliated TaS2.