Nanoparticle based devices are becoming of great interest because of their single-electron transport behavior, and high surface charge density. Nanoparticle based devices operate at low power, and are potentially highly stable and extremely robust. Making interconnections to nanoparticle devices, however, has been an impending issue. Also percolating/conductive array of nanoparticles is not easy to build since repulsion between the charged nanoparticles causes them to deposit at distance significantly larger for electron tunneling. In this study, we resolve these challenges to make nanoparticle based electronic devices. Using biological (bacteria) or physical (polyelectrolyte fiber) scaffolds, we selectively deposited percolating array of 30 nm Au nanoparticles, to produce a highly versatile nanoparticle-organic hybrid device. The device is based on electron tunneling phenomena, which is highly sensitive to change in inter-particle distance and dielectric constant between nanoparticles. The key to building this structure is the molecular brushes on the surface of the scaffold, which shield the charge on nanoparticle to allow for percolating deposition. The electrostatic attraction for such a deposition on bacteria was measured to be so strong (0.038 N/m) that it could bend a 400 nm long and 25 nm wide gold nanorod. Once the device is built, the hygroscopic scaffolds were actuated by humidity, to modulate the electron tunneling barrier width (or height) between the metallic nanoparticles. A decrease in inter-particle separation by < 0.2 nm or a change in the dielectric constant from ~ 40 to 3 (for humidity excursion from 20% to ~0%), causes a 40-150 fold increase in electron tunneling current. The coupling between the underlying scaffold and the Au particle structure is essential to achieving such a high and robust change in current. In contrast to most humidity sensors, the sensitivity is extremely high at low humidity. This device is >10-fold better than standard microelectronic and MEMS technology based humidity sensors. After the deposition, the "live" bacterial scaffold retains its biological construct, providing an avenue for active integration of biological functions with electronic transport in nanoparticle device. Such hybrids will be the key to conceptually new electronic devices that can be integrated with power and function of microorganisms, on flexible plastic-like substrates using simple beaker chemistry. The technology has broad potential based on variety of nanoparticles (for example, magnetic, metallic and semi-conducting) to make electro-optical and inorganic devices, bringing a prominent advancement in the present technology. Our work is published in, Angewandte Chemie, JACS and Nano Letters, and featured in places such as, Discover Magazine, Science News and Nature. / Ph. D.

Dielectrics have been an integral part of the electron devices and will likely resume playing a significant role in the future of nanoelectronics. An important step in assessing graphene potential as an alternative channel material for future electron devices is to benchmark its transport characteristics when integrated with dielectrics. Using back-gated and dual gated graphene field-effect transistors with top high-k metal-oxide dielectric, we study the dielectric thickness dependence of the carrier mobility. We show the carrier mobility decreases after deposition of metal-oxide dielectrics by atomic layer deposition (ALD) thanks to the Coulomb scattering by charged point defects in the dielectric. We investigate a novel method for the ALD of metal-oxide dielectrics on graphene, using an ultrathin nucleation layer that enables the realization of graphene field-effect transistors with aggressively scaled gate dielectric thickness. We show the nucleation layer significantly affects the quality of the subsequently deposited dielectric. In addition, we study transport characteristics of double layer systems. We demonstrate heterostructures consisting of two rotationally aligned bilayer graphene with an ultra-thin hexagonal boron nitride dielectric in between fabricated using advanced layer-by-layer transfer as well as layer pickup techniques. We show that double bilayer graphene devices possess negative differential resistance and resonant tunneling in their interlayer current-voltage characteristics in a wide range of temperatures. We show the resonant tunneling occurs either when the charge neutrality points of the two bilayer graphene are energetically aligned or when the lower conduction sub-band of one layer is aligned with the upper conduction sub-band of the opposite layer. Finally, we study the Raman spectra and the magneto-transport characteristics of A-B stacked and rotationally misaligned bilayer graphene deposited by chemical-vapor-deposition (CVD) on Cu. We show that the quantum Hall states (QHSs) sequence of the CVD grown A-B stacked bilayer graphene is consistent with that of natural bilayer graphene, while the sequence of the QHSs in the CVD grown rotationally misaligned bilayer graphene is a superposition of monolayer graphene QHSs. From the magnetotransport measurements in rotationally misaligned CVD-grown bilayer we determine the layer densities and the interlayer capacitance. / text

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The scanning tunneling microscope (STM) based on quantum tunneling can attain atomic-scale spatial resolution and help elucidate a wealth of phenomena in the microscopic world. However a limitation in scanning tunneling microscopy is the low temporal resolution due to readout circuit frequency rolloff at a few kHz. This limitation can be overcome by using downmixing directly in the tunneling junction. With this technology we measure the high frequency vibrational modes (~ 1 MHz) of MEMS doubly-clamped beams and explore the implication of STM downmixing for nanomechanics.

Resultados do Grupo de Renormalização Numérico (GRN) para a condutância linear dependente da temperatura associada a corrente de tunelamento através de uma ponta de prova nas proximidades de uma impureza magnética são apresentados. Nós usamos o Modelo de Anderson de uma impureza para descrever o metal hospedeiro e um Hamiltoniano livre para simular a ponta de prova do MVT (Microscópio de Varredura por Tunelamento). O cálculo da condutância é obtida a partir da fórmula de Kubo com o Hamiltoniano de tunelamento tratado como uma perturbação com dois canais de tunelamento, ponta-impureza e ponta-substrato, com o objetivo de descrever esse sistema que está totalmente fora do equilíbrio. Esse cálculo é guiado pelo GRN de Wilson para determinar a fórmula da condutância em termos de densidades espectrais: a densidade local da impureza e a densidade relativa ao primeiro sítio de condução da rede tight-binding do GRN. Esse resultado para o operador do GRN transforma esse objeto teórico em uma quantidade mensurável. Mostramos sob condições especiais, que o gráfico da condutância em função da temperatura é uma curva universal. Como função da posição ponta-impureza, as correntes de tunelamento mostram oscilações de Friedel, que determinam o tamanho da nuvem Kondo. Finalmente, mostramos como função da energia da impureza, a corrente da impureza para a ponta mostra um platô de Kondo. A interferência entre essa corrente e a que flui da banda de condução para a ponta exibe anti-ressonâncias de Fano como as observadas em medidas espectroscópicas. / Numerical Renormalization Group (NRG) results for the temperature dependent linear conductance associated with the scanning-tunneling current through a probe near a magnetic impurity are reported. We used the Single Impurity Anderson Model to describe the host metal and a free electron Hamiltonian to simulate a STM (Scanning Tunneling Microscope) biased tip. The calculation of the conductance is obtained from the Kubo Formula with the Tunneling Hamiltonian treated as a perturbation with two tunneling channels, STM tip-impurity and STM tip-host metal, with the objective to describe this fully nonequilibrium system. This calculation is guided by Wilson\'s NRG to determine a conductance formula as a funciton of spectral densities: the local impurity density and the density relative to the first conduction site of the NRG tight-binding chain. This result for the NRG operator transforms this theoretical object into a measurable quantity. We show that, under special conditions, plotted as a function of temperature, this zero-bias conductance follows a universal curve. As a function of tip-impurity separation, the tunneling currents display Friedel Oscilations, which determine the size of the Kondo cloud. Finally, plotted as a function of impurity energy, the current from the impurity to the tip displays a Kondo plateau. The inferference between this current and that flowing from the conduction band to the tip displays Fano anti-ressonances analogous to those seen in spectroscopic measurements.

In this thesis I concentrate on the description of electron transport properties of microscopic objects, including molecular junctions and nano junctions, in particular, inelastic electron tunneling in surface-adsorbate systems are examined with more contemplations. Boosted by the rapid advance in experimental techniques at the microscopic scale, various electric experiments and measurements sprung up in the last decade. Electric devices, such as transistors, switches, wires, etc. are expected to be integrated into circuit and performing like traditional semiconductor integrated circuit (IC). On the other hand, detailed information about transport properties also provides new physical observable quantities to characterize the systems. For molecular electronics, which is in the state of growing up, its further applications demands more thorough understanding of the underlying mechanism, for instance, the effects of molecular configuration and conformation, inter- or intra-molecular interactions, molecular-substrate interactions, and so on. Inelastic electron tunneling spectroscopy (IETS), which reflects vibration features of the system, is also a finger print property, and can thus be employed to afford the responsibility of single molecular identification with the help of other experimental techniques and theoretical simulations.There are two parts of work presented in this thesis, the first one is devoted to the calculation of electron transport properties of molecular or nano junctions: we have designed a negative differential resistance (NDR) device based on graphene nanoribbons (GNRs), where the latter is a star material in scientific committee since its birth;The transport properties of DNA base-pair junctions are also examined by theoretical calculation, relevant experimental results on DNA sequencing have been explained and detailed issues are suggested.The second part focused on the simulation of scanning tunneling microscope mediated IETS (STM-IETS). We have implemented a numerical scheme to calculate the inelastic tunneling intensity based on Tersoff-Hamann approximation and finite difference method, benchmark results agree well with experimental and previous theoretical ones; Two applications of single molecular chemical identification are also presented following benchmarking. / QC20100630

Common to molecular electronics studies, nanoscale break junctions created through electromigration also naturally produce electroluminescent arrays of individual gold nanoclusters spanning the electrodes. Due to inelastic electron tunneling into cluster electronic energy levels, these several-atom nanoclusters (Au~18-22) exhibit bright, field-dependent, antibunched emission in the near infrared (650800 nm), acting as room-temperature electrically driven single-photon sources. AC electrical excitation with time-stamping of photon arrival times enables fast and local tracking of electrode-nanocluster coupling dynamics demonstrating that charge injection to the clusters is directly modulated by dynamic coupling to individual electrodes. The electrode-nanocluster coupling rate fluctuates by nearly an order of magnitude and, due to the asymmetry of the electromigration process, exhibits preferential charge injection from the anode. Directly reporting on (and often facilitating) nanoscale charge transport, time-tagged single-molecule electroluminescence reveals a significant mechanism for nanoscale charge transport in nanoscale gold break junctions, and offers direct readout of the electrode-molecule interactions that can be correlated with current flow. Single-molecule electroluminescence techniques for characterization of electrode heterogeneity and dynamics as well as implications for future research are discussed.

The work presented in this thesis shows how tunneling spectroscopy techniques can be applied to metallic nanoparticles to obtain useful information about fundamental physical processes in nanoscopic length scales. At low temperatures, the discrete character of the energy spectrum of these particles, allows the study of spin-polarized current via resolved "electron-in-a-box" energy levels. In samples consisting of two ferromagnetic electrodes tunnel coupled to single aluminum nanoparticles, spin accumulation mechanisms are responsible for the observed spin-polarized current. The observed effect of an applied perpendicular magnetic field, relative to the magnetization orientation of the electrodes, indicates the suppression of spin precession in such small particles. More generally, in the presence of an external non-collinear magnetic field, it is the local field "felt" by the particle that determines the character of the tunnel current. This effect is also observed in the case where only one of the electrodes is ferromagnetic. In contrast to the non-magnetic case, ferromagnetic nanoparticles exhibit a much more complex energy spectrum, which cannot be accounted for, using the simple free-electron picture. It will be shown that interactions between quasi-particle excitations due to sequential electron tunneling and spin excitations in the particle are likely to play an important role in the observed temperature/voltage dependence of magnetic hysteresis loops.

<p>A theoretical framework for calculating electron transport through molecular junctions is presented. It is based on scattering theory using a Green's function formalism. The model can take both elastic and inelastic scattering into account and treats chemical and physical bonds on equal footing. It is shown that it is quite reliable with respect to the choice of functional and basis set. Applications concerning both elastic and inelastic transport are presented, though the emphasis is on the inelastic transport properties. The elastic scattering application part is divided in two part. The first part demonstrates how the current magnitude is strongly related to the junction width, which provides an explanation why experimentalists get two orders of magnitude differences when performing measurements on the same type of system. The second part is devoted to a study of how hydrogenbonding affects the current-voltage (I-V) characteristics. It is shown that for a conjugated molecule with functional groups, the effects can be quite dramatic. This shows the importance of taking possible intermolecular interactions into account when evaluating and comparing experimental data. The inelastic scattering part is devoted to get accurate predictions of inelastic electron tunneling spectroscopy (IETS) experiments. The emphasis has been on elucidating the importance of various bonding conditions for the IETS. It is shown that the IETS is very sensitive to the shape of the electrodes and it can also be used to discriminate between different intramolecular conformations. Temperature dependence is nicely reproduced. The junction width is shown to be of importance and comparisons between experiment as well as other theoretical predictions are made.</p>

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