Tunneling current modulation in atomically precise graphene nanoribbon heterojunctions

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates

Atomically inspired k · p approach and valley Zeeman effect in transition metal dichalcogenide monolayers

International audienceWe developed a six-band k · p model that describes the electronic states of monolayer transition metal dichalcogenides (TMDCs) in K-valleys. The set of parameters for the k · p model is uniquely determined by decomposing tight-binding (TB) models in the vicinity of K ±-points. First, we used TB models existing in literature to derive systematic parametrizations for different materials, including MoS2, WS2, MoSe2 and WSe2. Then, by using the derived six-band k · p Hamiltonian we calculated effective masses, Landau levels, and the effective exciton g-factor g X 0 in different TMDCs. We showed that TB parameterizations existing in literature result in small absolute values of g X 0 , which are far from the experimentally measured g X 0 ≈ −4. To further investigate this issue we derived two additional sets of k · p parameters by developing our own TB parameterizations based on simultaneous fitting of ab-initio calculated, within the density functional (DFT) and GW approaches, energy dispersion and the value of g X 0. We showed that the change in TB parameters, which only slightly affects the dispersion of higher conduction and deep valence bands, may result in a significant increase of |g X 0 |, yielding close-to-experiment values of g X 0. Such a high parameter sensitivity of g X 0 opens a way to further improvement of DFT and TB models

Phonon contribution to electrical resistance of acceptor-doped single-wall carbon nanotubes assembled into transparent films

The electrical resistance of pristine and acceptor-doped single-wall carbon nanotubes assembled into transparent films was measured in the temperature range of 5 to 300 K. The doping was accomplished by filling the nanotubes with iodine or CuCl from the gas phase. After doping the films resistance appeared to drop down by one order of magnitude, to change the nonmonotonic temperature behavior, and to reduce the crossover temperature. The experimental data have been perfectly fitted in frames of the known heterogeneous model with two contributions: from the nanotube bundles (with quasi-one-dimensional conductivity) and from the interbundle electron tunneling. The doping was observed to decrease the magnitudes of both contributions. In this paper we have revealed the main reason of changes in the nanotube part. It is considered to be connected with the involvement of low-energy phonons, which start to participate in the intravalley scattering due to the shift of the Fermi level after doping. The values of the Fermi level shift into the valence band are estimated to be equal to -0.6 eV in the case of iodine doping and -0.9 eV in the case of CuCl doping. These values are in qualitative agreement with the optical absorption data.Peer reviewe

Charge transport mechanisms in macro-scale CNT films

Carbon nanotubes (CNT) attract considerable attention due to their unique physical properties and potential application in optoelectronics. Despite of intensive studies there is still a lack of agreement in experimental data on electrical properties of the material. Here we report on extremely broad-band conductivity and dielectric permittivity spectra of macro-scale thin films composed of large number of randomly distributed pristine and p-doped CNTs of different length, measured in the frequency range 5-24 000 cm-1 and at temperatures from 5 to 300 K. We show that terahertz-infrared spectra of the films are determined by response of delocalized charge carriers. Controversially to the existing experimental results we did not clearly observe the so-called terahertz conductivity peak. Yet, a weak bump-like feature in conductivity spectra around 30 cm-1 showed no signs of tube length dependence. We associate its origin with plasmonic excitation due to reflections of charge carrier plasma at the CNT intersections. Applying the Drude-model to describe the low frequency conductivity and dielectric permittivity spectra of CNT films we obtained effective values of carries parameters. Our results can shed light on electromagnetic waves absorption mechanisms and will be useful while designing new CNT-based devices.Peer reviewe

Quasi-two-dimensional thermoelectricity in SnSe

Stannous selenide is a layered semiconductor that is a polar analog of black phosphorus and of great interest as a thermoelectric material. Unusually, hole doped SnSe supports a large Seebeck coefficient at high conductivity, which has not been explained to date. Angle-resolved photoemission spectroscopy, optical reflection spectroscopy, and magnetotransport measurements reveal a multiple-valley valence-band structure and a quasi-two-dimensional dispersion, realizing a Hicks-Dresselhaus thermoelectric contributing to the high Seebeck coefficient at high carrier density. We further demonstrate that the hole accumulation layer in exfoliated SnSe transistors exhibits a field effect mobility of up to 250 cm(2)/V s at T = 1.3K. SnSe is thus found to be a high-quality quasi-two-dimensional semiconductor ideal for thermoelectric applications

Tunneling current modulation in atomically precise graphene nanoribbon heterojunctions

Lateral heterojunctions of atomically precise graphene nanoribbons GNRs hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi metallic and wide bandgap GNRs, and report characterization by scanning tunneling microscopy, angle resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide bandgap GNR segments. The current voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbate