Scaling of silicon (Si) transistors is predicted to fail below 5-nanometer (nm) gate lengths because of severe short channel effects. As an alternative to Si, certain layered semiconductors are attractive for their atomically uniform thickness down to a monolayer, lower dielectric constants, larger band gaps, and heavier carrier effective mass. Here, we demonstrate molybdenum disulfide (MoS) transistors with a 1-nm physical gate length using a single-walled carbon nanotube as the gate electrode. These ultrashort devices exhibit excellent switching characteristics with near ideal subthreshold swing of ~65 millivolts per decade and an On/Off current ratio of ~10 Simulations show an effective channel length of ~3.9 nm in the Off state and ~1 nm in the On state.

Advanced beyond-silicon electronic technology requires discoveries of both new channel materials and ultralow-resistance contacts 1,2 . Atomically thin two-dimensional (2D) semiconductors have great potential for realizing high-performance electronic devices 1,3 . However, because of metal-induced gap states (MIGS) 4-7 , energy barriers at the metalsemiconductor interface, which fundamentally lead to high contact resistances and poor current-delivery capabilities, have restrained the advancement of 2D semiconductor transistors to date 2,8,9 . Here, we report a novel ohmic contact technology between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where MIGS is sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a record-low contact resistance (R C ) of 123 Ω μm, and a recordhigh on-state current density (I ON ) of 1135 µA µm -1 on monolayer MoS 2 . We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS 2 , WS 2 , and WSe 2 . Our reported R C values are a significant improvement for 2D semiconductors, and approaching the quantum limit. This technology unveils the full potential of high-performance monolayer transistors that are on par with the state-of-the-art 3D semiconductors, enabling further device down-scaling and extending Moore's Law.The electrical contact resistance at a metal-semiconductor (M-S) interface has been an increasingly critical, yet unsolved issue for the semiconductor industry, hindering the ultimate

Here we demonstrate Au nanoparticle self-similar chain structure organized by triangle DNA origami with well-controlled orientation and <10 nm spacing. We show for the first time that a large DNA complex (origami) and multiple AuNP conjugates can be well-assembled and purified with reliable yields. The assembled structure could be used to generate high local-field enhancement. The same method can be used to precisely localize multiple components on a DNA template for potential applications in nanophotonic, nanomagnetic, and nanoelectronic devices.

We report the direct observation of the thermalization of electrons in gold following 180 fs optical pulse excitation. The evolution of the electron energy distribution from the nascent (as photoexcited) to a hot Fermi-Dirac distribution was measured by time-resolved photoemission spectroscopy. Depending on the excitation density, thermalization times as long as =1 ps were observed. A model incorporating both electron-electron and electron-phonon scattering, and using Fermi-liquid theory to properly account for screening is found to reproduce the main features of the experiment.Electron-electron (e-e) scattering in metals has usually been studied by transport' measurements. The contribution of e-e scattering to resistance can only be observed at low temperature, because above the Debye temperature electron-phonon (e-p) scattering completely dominates the resistivity.According to Landau s Fermi-liquid theory, the resistance due to e-e scattering is p, , = AT, where T is the temperature and A is a constant. However, even at low temperature, extraction of p, , from the measured resistivity is complicated by electron-phonon (e-p) and defect scattering. ' Observation of the thermalization of electrons excited by ultrafast optical pulses provides an alternative means to study e-e scattering. The relaxation of an optically excited, non-Fermi-Dirac distribution to a hot Fermi-Dirac distribution is mainly through e-e scattering due to the large momentum exchange and large phase space available for the process which involves quasiparticle energies in the range of an electron volt.In this paper, we report the first direct measurement of the thermalization process in an optically excited metal. We are able to observe the nascent (as photoexcited) electron energy distribution, and the time evolution from the nascent distribution to a Fermi-Dirac distribution. The thermalization process is found to take up to -1 ps for low optical excitation levels, and proceeds more rapidly for higher optical excitation levels. Because thermalization and electron-phonon energy relaxation occur on similar time scales {on the order of ps), we find that even in this regime it is necessary to simultaneously include both e-e and e-p scattering to fully understand the dynamics. A model based on the Boltzmann transport equation under the relaxation-time approximation is pro-t=0 fs 130 fs 400 fs 670 fs 1300 fs 0.1 0 C ENERGY (eV) FIG. 1. Electron energy distribution function vs energy with 120 pJ/cm absorbed laser fluence at five time delays. The dashed line is the best Fermi-Dirac fit and the corresponding electron temperature T,, is shown. The vertical scale is in units of the density of states.posed to explain the experiment. Fermi-liquid theory is used to properly account for Coulomb screening. Time-resolved photoemission spectroscopy was used to measure the time evolution of the electron energy distribution following ultrashort laser pulse excitation of a gold sample. The sample was a room temperature 300-A-thick gold film held in vacuum at 5X10...

Temperature-dependent I-V and C-V spectroscopy of single InAs nanowire field-effect transistors were utilized to directly shed light on the intrinsic electron transport properties as a function of nanowire radius. From C-V characterizations, the densities of thermally activated fixed charges and trap states on the surface of untreated (i.e., without any surface functionalization) nanowires are investigated while enabling the accurate measurement of the gate oxide capacitance, therefore leading to the direct assessment of the field-effect mobility for electrons. The field-effect mobility is found to monotonically decrease as the radius is reduced to <10 nm, with the low temperature transport data clearly highlighting the drastic impact of the surface roughness scattering on the mobility degradation for miniaturized nanowires. More generally, the approach presented here may serve as a versatile and powerful platform for in-depth characterization of nanoscale, electronic materials.

Bottom-up synthesized graphene nanoribbons and graphene nanoribbon heterostructures have promising electronic properties for high-performance field-effect transistors and ultra-low power devices such as tunneling field-effect transistors. However, the short length and wide band gap of these graphene nanoribbons have prevented the fabrication of devices with the desired performance and switching behavior. Here, by fabricating short channel (L ch ~ 20 nm) devices with a thin, high-κ gate dielectric and a 9-atom wide (0.95 nm) armchair graphene nanoribbon as the channel material, we demonstrate field-effect transistors with high on-current (I on > 1 μA at V d = −1 V) and high I on /I off ~ 105 at room temperature. We find that the performance of these devices is limited by tunneling through the Schottky barrier at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high-performance short-channel field-effect transistors with bottom-up synthesized armchair graphene nanoribbons.

Direct deposition of graphene on various dielectric substrates is demonstrated using a single-step chemical vapor deposition process. Single-layer graphene is formed through surface catalytic decomposition of hydrocarbon precursors on thin copper films predeposited on dielectric substrates. The copper films dewet and evaporate during or immediately after graphene growth, resulting in graphene deposition directly on the bare dielectric substrates. Scanning Raman mapping and spectroscopy, scanning electron microscopy, and atomic force microscopy confirm the presence of continuous graphene layers on tens of micrometer square metal-free areas. The revealed growth mechanism opens new opportunities for deposition of higher quality graphene films on dielectric materials.

O rganic-based photovoltaic cells (OPVs) are of great interest owing to their potential for low-cost solar energy conversion. 1 An important breakthrough for OPVs was the use of a heterojunction (HJ) structure, in which the difference of the energy levels of two materials (donor and acceptor) can lead to efficient dissociation of photogenerated excitons at the HJ interfaces. 1 Since then, tremendous efforts have been taken to optimize the carrier donor/acceptor (DA) interface morphology to improve the photogenerated exciton dissociation and consequently the overall power conversion efficiency. 2 One successful approach is to use a bulk heterojunction (BHJ) structure that can create dissociation centers everywhere within the active layer. 2 Typically, the formation of a BHJ structure can be achieved via self-assembly of nanostructured soft materials by spontaneous phase separation in a number of solution processed polymer/ fullerene systems, yet efficiency in these structures might be significantly reduced through unpredicted shunt paths and isolated islands of materials. 3 Nanoimprint lithography (NIL) offers a potential solution for producing well-defined interpenetrating networks at the DA interface and is compatible with roll-to-roll manufacturing for lowcost and high-throughput nanopatterning. 4,5 To efficiently harvest photogenerated excitons, densely packed nanoimprinted OPV structures with halfpitch smaller than 2 times that of the exciton diffusion length are needed (typically sub-20 nm regime). 6 Recent efforts in this field have been mainly focusing on polymeric PV materials. However, OPVs with small-molecular weight materials could also benefit from similar morphologies. In addition, small-molecular weight OPV materials provide additional advantages over polymers, such as higher chemical/thermal stability and higher material purity. 7 Previous work has shown relatively poor stability of imprinted nanostructures in smallmolecular compounds. 8Ϫ10 Problems arise due to pronounced surface diffusion and self-faceting and are exacerbated when features head toward the sub-20 nm regime. 11,12 These instabilities must be understood and overcome to achieve efficient nanostructured OPVs.Boron subphthalocyanines (SubPcs) are a class of photoactive small-molecularweight materials with unique physical properties. 13 A typical SubPc has a nonplanar pyramid-shaped structure, in which the boron atom is surrounded by three coupled