Vibrational and electronic heating in nanoscale junctions.

Understanding and controlling the flow of heat is a major challenge in nanoelectronics. When a junction is driven out of equilibrium by light or the flow of electric charge, the vibrational and electronic degrees of freedom are, in general, no longer described by a single temperature. Moreover, characterizing the steady-state vibrational and electronic distributions in situ is extremely challenging. Here, we show that surface-enhanced Raman emission may be used to determine the effective temperatures for both the vibrational modes and the electrons in the current in a biased metallic nanoscale junction decorated with molecules. Molecular vibrations show mode-specific pumping by both optical excitation and d.c. current, with effective temperatures exceeding several hundred kelvin. Anti-Stokes electronic Raman emission indicates that the effective electronic temperature at bias voltages of a few hundred millivolts can reach values up to three times the values measured when there is no current. The precise effective temperatures are model-dependent, but the trends as a function of bias conditions are robust, and allow direct comparisons with theories of nanoscale heating.

Two-terminal molecular memories from solution-deposited C60 films in vertical silicon nanogaps.

We demonstrate here two-terminal, charge-based memory from C60 films inside vertical 7 nm silicon nanogap devices. This testbed structure eliminated the possibility of metal migration in the nanostructure because the two electrodes are made solely of silicon; hence, the often troublesome and confusing possibility of filamentary metal formation is obviated. Saturated solutions of C60 in toluene, mesitylene, and 1-methylnaphthalene were each used to deposit these films at elevated temperatures. Electrical I-V measurements reveal a high yield (67%) of devices demonstrating bipolar, switchable hysteresis from both the mesitylene- and 1-methylnaphthalene-deposited devices, while the toluene-grafted devices display no such behavior. Pulse-based memory measurements of switching devices indicate high ON/OFF ratios (maximum approximately 1500), good stability (>100 cycles without device degradation) for molecular devices, and low operating currents (approximately 10(-11) A) in room temperature testing.

Kinetics of diazonium functionalization of chemically converted graphene nanoribbons.

We demonstrate that graphene nanoribbons (GNRs) produced by the oxidative unzipping of carbon nanotubes can be chemically functionalized by diazonium salts. We show that functional groups form a thin layer on a GNR and modify its electrical properties. The kinetics of the functionalization can be monitored by probing the electrical properties of GNRs, either in vacuum after the grafting, or in situ in the solution. We derive a simple kinetics model that describes the change in the electrical properties of GNRs. The reaction of GNRs with 4-nitrobenzene diazonium tetrafluoroborate is reasonably fast, such that >60% of the maximum change in the electrical properties is observed after less than 5 min of grafting at room temperature.

Controllable molecular modulation of conductivity in silicon-based devices.

The electronic properties of silicon, such as the conductivity, are largely dependent on the density of the mobile charge carriers, which can be tuned by gating and impurity doping. When the device size scales down to the nanoscale, routine doping becomes problematic due to inhomogeneities. Here we report that a molecular monolayer, covalently grafted atop a silicon channel, can play a role similar to gating and impurity doping. Charge transfer occurs between the silicon and the molecules upon grafting, which can influence the surface band bending, and makes the molecules act as donors or acceptors. The partly charged end-groups of the grafted molecular layer may act as a top gate. The doping- and gating-like effects together lead to the observed controllable modulation of conductivity in pseudometal-oxide-semiconductor field-effect transistors (pseudo-MOSFETs). The molecular effects can even penetrate through a 4.92-mum thick silicon layer. Our results offer a paradigm for controlling electronic characteristics in nanodevices at the future diminutive technology nodes.

Silicon/molecule interfacial electronic modifications.

Electronic structures at the silicon/molecule interface were studied by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, inverse photoemission spectroscopy, and Kelvin probe techniques. The heterojunctions were fabricated by direct covalent grafting of a series of molecules (-C6H4-X, with X = NMe2, NH2, NO2, and Mo6 oxide cluster) onto the surface of four types of silicon substrates (both n- and p-type with different dopant densities). The electronic structures at the interfaces were thus systematically tuned in accordance with the electron-donating ability, redox capability, and/or dipole moment of the grafted molecules. The work function of each grafted surface is determined by a combination of the surface band bending and electron affinity. The surface band bending is dependent on the charge transfer between the silicon substrate and the grafted molecules, whereas electron affinity is dependent on the dipole moment of the grafted molecules. The contribution of each to the work function can be separated by a combination of the aforementioned analytical techniques. In addition, because of the relatively low molecular coverage on the surface, the contribution from the unreacted H-terminated surface to the work function was considered. The charge-transfer barrier of silicon substrates attached to different molecules exhibits a trend analogous to surface band bending effects, whereas the surface potential step exhibits properties analogous to electron affinity effects. These results provide a foundation for the utilization of organic molecule surface grafting as a means to tune the electronic properties of semiconductors and, consequently, to achieve controllable modulation of electronic characteristics in small semiconductor devices at future technology nodes.

Viscosity dependence of the denitrogenation quantum yield in azoalkane photolysis: experimental evidence for reversible formation of the diazenyl diradical.

[reaction: see text] Experimental evidence is reported for the reversible formation of the singlet diazenyl diradical ((1)DZ), photolytically generated from the structurally elaborate DBH-type azoalkane. Reversiblity of the (1)DZ formation manifests itself through the decrease of the photodenitrogenation quantum yield over a ca. 40-fold viscosity variation (from 0.5 to 19.3 cP). This viscosity behavior is interpreted in terms of frictional effects on the competitive reaction modes of the diazenyl diradical.