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We used multiprobe scanning tunneling microscope (STM) to fabricate and electrically characterize nanostructures on Si surfaces. We overcame resistive contacts by using field evaporation to clean tip apexes in order to create Ohmic contact with the Si surface states on a Si substrate. A two-probe (2P-) STM with Ohmic contact allowed for measurement at very low bias, limiting conduction through space-charge layer and bulk states. The Ohmic 2P-STM measurement clarified the surface conductivity of the Si(111)-(7 × 7) surface. We also confirmed that Ohmic 2P-STM can be replaced with more convenient Ohmic one-probe STM for the conductance measurements on the Si surface. We prepared nanostructures using STM lithography to define electronically isolated two-dimensional (2D) regions with various aspect ratios. Their surface conduction properties are described well by the conventional sheet model, proving the diffusive 2D conduction on the Si surface. Constrictions and breaks in 2D structures were also evaluated. Ohmic 2P-STM will be helpful for the investigation of exploratory atomic-scale circuitry or cutting-edge materials sciences.

This paper introduces SiQAD, a computer-aided design tool enabling the rapid design and simulation of atomic silicon dangling bond quantum dot patterns capable of computational logic. Several simulation tools are included, each able to inform the designer on various aspects of their designs: a ground-state electron configuration finder, a non-equilibrium electron dynamics simulator, and an electric potential landscape solver with clocking electrode support. Simulations have been compared against past experimental results to inform the electron population estimation and dynamic behavior. New logic gates suitable for this platform have been designed and simulated, and a clocked wire has been demonstrated. This work paves the way for the exploration of the vast and fertile design space of atomic silicon dangling bond quantum dot circuits.

Using a non-contact atomic force microscope (nc-AFM), we examine continuous dangling bond (DB) wire structures patterned on the hydrogen terminated silicon (100)-2 × 1 surface. By probing the DB structures at varying energies, we identify the formation of previously unobserved ionic charge distributions which are correlated to the net charge of DB wires and their predicted degrees of freedom in lattice distortions. Performing spectroscopic analysis, we identify higher energy configurations corresponding to alternative lattice distortions as well as tip-induced charging effects. By varying the length and orientation of these DB structures, we further highlight key features in the formation of these ionic surface phases.

A simple and cost effective method to fabricate multiple tungsten (W) single atom tips (SATs) from both poly and single crystalline wires is reported. Two or four tips attached to a holder are electrochemically etched together in NaOH solution followed by a controlled field assisted reactive gas etching in vacuum using nitrogen as an etching gas and helium as an imaging gas. A Common high voltage is applied simultaneously to all nanotips to shape the apexes towards single atoms. Single atom tips were achieved for both W(111) and W(110) while trimer tips were also achieved for W(111). This observation can lead to an important step towards realizing simplified etching processes of multiple tips which in turn can help to simultaneously fabricate numerous tips leading to mass fabrication and characterization.

At the atomic scale, there has always been a trade-off between the ease of fabrication of structures and their thermal stability. Complex structures that are created effortlessly often disorder above cryogenic conditions. Conversely, systems with high thermal stability do not generally permit the same degree of complex manipulations. Here, we report scanning tunneling microscope (STM) techniques to substantially improve automated hydrogen lithography (HL) on silicon, and to transform state-of-the-art hydrogen repassivation into an efficient, accessible error correction/editing tool relative to existing chemical and mechanical methods. These techniques are readily adapted to many STMs, together enabling fabrication of error-free, room-temperature stable structures of unprecedented size. We created two rewriteable atomic memories (1.1 petabits per in2), storing the alphabet letter-by-letter in 8 bits and a piece of music in 192 bits. With HL no longer faced with this trade-off, practical silicon-based atomic-scale devices are poised to make rapid advances towards their full potential., Manipulation at the atomic scale comes with a trade-off between simplicity and thermal stability. Here, Achal et al. demonstrate improved automated hydrogen lithography and repassivation, enabling error-corrected atomic writing of large-scale structures/memories that are stable at room temperature.

Atomic-scale characterization and manipulation with scanning probe microscopy rely upon the use of an atomically sharp probe. Here we present automated methods based on machine learning to automatically detect and recondition the quality of the probe of a scanning tunneling microscope. As a model system, we employ these techniques on the technologically relevant hydrogen-terminated silicon surface, training the network to recognize abnormalities in the appearance of surface dangling bonds. Of the machine learning methods tested, a convolutional neural network yielded the greatest accuracy, achieving a positive identification of degraded tips in 97% of the test cases. By using multiple points of comparison and majority voting, the accuracy of the method is improved beyond 99%.

Many diverse material systems are being explored to enable smaller, more capable and energy efficient devices. These bottom up approaches for atomic and molecular electronics, quantum computation, and data storage all rely on a well-developed understanding of materials at the atomic scale. Here, we report a versatile scanning tunneling microscope (STM) charge characterization technique, which reduces the influence of the typically perturbative STM tip field, to develop this understanding even further. Using this technique, we can now observe single molecule binding events to atomically defined reactive sites (fabricated on a hydrogen-terminated silicon surface) through electronic detection. We then developed a simplified error correction tool for automated hydrogen lithography, quickly directing molecular hydrogen binding events using these sites to precisely repassivate surface dangling bonds (without the use of a scanned probe). We additionally incorporated this molecular repassivation technique as the primary rewriting mechanism in ultradense atomic data storage designs (0.88 petabits per in²).

With nanoelectronics reaching the limit of atom-sized devices, it has become critical to examine how irregularities in the local environment can affect device functionality. Here, we characterize the influence of charged atomic species on the electrostatic potential of a semiconductor surface at the subnanometer scale. Using noncontact atomic force microscopy, two-dimensional maps of the contact potential difference are used to show the spatially varying electrostatic potential on the (100) surface of hydrogen-terminated highly doped silicon. Three types of charged species, one on the surface and two within the bulk, are examined. An electric field sensitive spectroscopic signature of a single probe atom reports on nearby charged species. The identity of one of the near-surface species has been uncertain in the literature, and we suggest that its character is more consistent with either a negatively charged interstitial hydrogen or a hydrogen vacancy complex.

Multi-probe scanning tunneling microscopy can play a role in various electrical measurements and characterization of nanoscale objects. The consistent close placement of multiple probes relies on very sharp apexes with no other interfering materials along the shank of the tip. Electrochemically etched tips can prepare very sharp apex tips; however, other asperities on the shank can cause interference and limit the close positioning of multiple tips to beyond the measured radii. Gallium focused ion beam (FIB) milling is used to remove any interfering material and allow closely spaced tips with a consistent yield. The tip apex radius is evaluated with field ion microscopy, and the probe spacing is evaluated with STM on hydrogen terminated silicon surfaces. FIB prepared tips can consistently achieve the measured probe to probe spacing distances of 25 nm–50 nm.

As the development of atom scale devices transitions from novel, proof-of-concept demonstrations to state-of-the-art commercial applications, automated assembly of such devices must be implemented. Here we present an automation method for the identification of defects prior to atomic fabrication via hydrogen lithography using deep learning. We trained a convolutional neural network to locate and differentiate between surface features of the technologically relevant hydrogen-terminated silicon surface imaged using a scanning tunneling microscope. Once the positions and types of surface features are determined, the predefined atomic structures are patterned in a defect-free area. By training the network to differentiate between common defects we are able to avoid charged defects as well as edges of the patterning terraces. Augmentation with previously developed autonomous tip shaping and patterning modules allows for atomic scale lithography with minimal user intervention.

We report on tuning the carrier capture events at a single dangling bond (DB) midgap state by varying the substrate temperature, doping type, and doping concentration. All-electronic time-resolved scanning tunneling microscopy (TR-STM) is employed to directly measure the carrier capture rates on the nanosecond time scale. A characteristic negative differential resistance (NDR) feature is evident in the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements of DBs on both n- and p-type doped samples. We find that a common model accounts for both observations. Atom-specific Kelvin probe force microscopy (KPFM) measurements confirm the energetic position of the DB's charge transition levels, corroborating STS studies. We show that under different tip-induced fields the DB can be supplied with electrons from two distinct reservoirs: the bulk conduction band and/or the valence band. We measure the filling and emptying rates of the DBs in the energy regime where electrons are supplied by the bulk valence band. We show that adding point charges in the vicinity of a DB shifts observed STS and NDR features due to Coulombic interactions.

We studied the charging behavior of an amorphous carbon thin film kept at liquid-nitrogen temperature under focused electron-beam irradiation. Negative charging of the thin film is observed. The charging is attributed to a local change in the work function of the thin film induced by electron-stimulated desorption similar to the working principle of the hole free phase plate in its Volta potential implementation at elevated temperature. The negative bias of the irradiated film arises from the electron beam induced desorption of water molecules from the carbon film surface. The lack of positive charging, which is expected for non-conductive materials, is explained by a sufficient electrical conductivity of the carbon thin film even at liquid-nitrogen temperature as proven by multi-probe scanning tunneling microscopy and spectroscopy measurements.

Using a noncontact atomic force microscope, we track and manipulate the position of single electrons confined to atomic structures engineered from silicon dangling bonds on the hydrogen terminated silicon surface. An attractive tip surface interaction mechanically manipulates the equilibrium position of a surface silicon atom, causing rehybridization that stabilizes a negative charge at the dangling bond. This is applied to controllably switch the charge state of individual dangling bonds. Because this mechanism is based on short range interactions and can be performed without applied bias voltage, we maintain both site-specific selectivity and single-electron control. We extract the short range forces involved with this mechanism by subtracting the long range forces acquired on a dimer vacancy site. As a result of relaxation of the silicon lattice to accommodate negatively charged dangling bonds, we observe charge configurations of dangling bond structures that remain stable for many seconds at 4.5 K. Subsequently, we use charge manipulation to directly prepare the ground state and metastable charge configurations of dangling bond structures composed of up to six atoms.

It has been proposed that miniature circuitry will ultimately be crafted from single atoms. Despite many advances in the study of atoms and molecules on surfaces using scanning probe microscopes, challenges with patterning and limited thermal structural stability have remained. Here we demonstrate rudimentary circuit elements through the patterning of dangling bonds on a hydrogen-terminated silicon surface. Dangling bonds sequester electrons both spatially and energetically in the bulk bandgap, circumventing short-circuiting by the substrate. We deploy paired dangling bonds occupied by one moveable electron to form a binary electronic building block. Inspired by earlier quantum dot-based approaches, binary information is encoded in the electron position, allowing demonstration of a binary wire and an OR gate. Rudimentary circuit elements, including a binary wire and an OR gate, can be created through the patterning of dangling bonds on a hydrogen-terminated silicon surface.

Quantum-dot cellular automata (QCA) provides a basis for classical computation without transistors. Many simulations of QCA rely upon the so-called intercellular Hartree approximation (ICHA), which neglects the possibility of entanglement between cells. The ICHA was originally proposed as a solution to the problem of exponential scaling in the computational cost of fully quantum mechanical treatments. However, in some cases, the ICHA predicted errors in QCA operation, and quantum correlations were required for circuits to operate correctly. While quantum correlations can remedy certain problems that present themselves in ICHA calculations, here we present simulations that show that quantum correlations may in fact be problematic in other situations, such as clocked QCA. Small groups of QCA cells are modelled with a Hamiltonian analogous to a quantum mechanical Ising-like spin chain in a transverse field, including the effects of intercellular entanglement completely. When energy relaxation is included in the model, we find that intercellular entanglement changes the qualitative behavior of the system, and new features appear. In clocked QCA, isolated groups of active cells have a tendency to oscillate between polarization states as information propagates. Additionally, energy relaxation tends to bring groups of cells to an unpolarized steady state. This contrasts with the results of previous simulations, which employed the ICHA. The ICHA may in fact be a good approximation in the limit of very low tunneling rates, which can be realized in lithographically defined quantum dots. However, in molecular and atomic implementations of QCA, entanglement will play a greater role. The degree to which intercellular correlations pose a problem for memory, and clocking depends upon implementation-specific details of the interaction of the system with its environment, as well as the system's internal dynamics.

Hydrogen scanning ion microscopy systems rely on nanotip gas field ion sources to generate the hydrogen ion beam. The exact structure of the nanotip and the applied electric field are shown to be important. It is demonstrated that hydrogen ion beams are found to occur as mixtures of H + , H 2 + and H 3 + depending on the electric field strength and the nanotip structure. Various nanotips were prepared, including single atom tips (SATs), trimers and other nano-structured tips to compare the contents of hydrogen ion beams. It was found that single atom tips produce primarily H 2 + at low operating voltages, but as the voltage is increased, H 3 + dominates. For the trimer case, H 2 + becomes a significant species and equals the H 3 + current but H 3 + can be isolated at higher voltages. For the hexamer tip structure, H 2 + almost completely dominates with little H 3 + being produced. H + is only observed in small quantities for all tip structures until a high voltage regime, where apex atom resolution is not observed. Comparisons W SATs and Ir SATs showed similar H 3 + /H 2 + product ratios indicating the nanotip structure plays a key role in the catalytic formation of H 3 + . Temperature affects are also discussed and operating parameters for single species ion beams are discussed.

The miniaturization of semiconductor devices to the scales where small numbers of dopants can control device properties requires the development of new techniques capable of characterizing their dynamics. Investigating single dopants requires sub-nanometer spatial resolution which motivates the use of scanning tunneling microscopy (STM), however, conventional STM is limited to millisecond temporal resolution. Several methods have been developed to overcome this shortcoming. Among them is all-electronic time-resolved STM, which is used in this work to study dopant dynamics in silicon with nanosecond resolution. The methods presented here are widely accessible and allow for local measurement of a wide variety of dynamics at the atomic scale. A novel time-resolved scanning tunneling spectroscopy technique is presented and used to efficiently search for dynamics.

A single line of dangling bonds (DBs) on Si(100)-2 × 1:H surface forms a perfect metallic atomic-wire. In this work, we investigate quantum transport properties of such dangling bond wires (DBWs) by a state-of-the-art first-principles technique. It is found that the conductance of the DBW can be gated by electrostatic potential and orbital overlap due to only a single DB center (DBC) within a distance of ∼16 Å from the DBW. The gating effect is more pronounced for two DBCs and especially, when these two DB “gates” are within ∼3.9 Å from each other. These effective length scales are in excellent agreement with those measured in scanning tunnelling microscope experiments. By analyzing transmission spectrum and density of states of DBC–DBW systems, with or without subsurface doping, for different length of the DBW, distance between DBCs and the DBW, and distance between DB gates, we conclude that charge transport in a DBW can be regulated to have both an on-state and an off-state using only one or two DBs.

We report the mechanically induced formation of a silicon-hydrogen covalent bond and its application in engineering nanoelectronic devices. We show that using the tip of a non-contact atomic force microscope (NC-AFM), a single hydrogen atom could be vertically manipulated. When applying a localized electronic excitation, a single hydrogen atom is desorbed from the hydrogen passivated surface and can be transferred to the tip apex as evidenced from a unique signature in frequency shift curves. In the absence of tunnel electrons and electric field in the scanning probe microscope junction at 0 V, the hydrogen atom at the tip apex is brought very close to a silicon dangling bond, inducing the mechanical formation of a silicon-hydrogen covalent bond and the passivation of the dangling bond. The functionalized tip was used to characterize silicon dangling bonds on the hydrogen-silicon surface, was shown to enhance the scanning tunneling microscope (STM) contrast, and allowed NC-AFM imaging with atomic and chemical bond contrasts. Through examples, we show the importance of this atomic scale mechanical manipulation technique in the engineering of the emerging technology of on-surface dangling bond based nanoelectronic devices., Comment: 9 pages (including references and Supplementary Section), 8 figures (5 in the main text, 3 in Supplementary)

Experiments aimed at assessing the robustness of nitrogen-etched, single-atom tips (SATs) prepared using W(1 1 1) single crystal wire were performed. Our experiments showed that single-atoms tips sustain minimal damage when exposed to atmospheric conditions and can be readily and quickly nitrogen-etched to single-atom tips thereafter. The SATs can be annealed at temperatures up to 1100 C with minimal shape changes. Moreover, annealing temperatures in excess of 1200 C resulted in an apex faceting which may prove important in further single-atom tip creation. Procedures for warming of the SATs from operating temperatures of 80 K were also evaluated to determine conditions that limit tip contamination. These results show that SATS could be fabricated in a dedicated vacuum system and subsequently transferred to other instruments where they would undergo a brief conditioning procedure to recover the single-atom apex configuration prior to being subjected to operating conditions.. © 2014 Elsevier B.V.

A procedure to control W(111) tip shape during etching to a single atom is described. It is demonstrated that the base of a single atom tip (SAT) can be shaped in order to alter the final operating voltage and emission opening angle of single atom tips for use as gas field ion sources or electron cold field emission sources. The operating voltages for single atom tips varied between 5 and 17 kV during helium ion beam generation. The emission properties of SATs were evaluated by fitting SAT images and measuring the full width at half maximum (FWHM) of the helium ion images. The FWHM is related to the linear opening angle and was evaluated as a function of SAT operating voltage. The results show that a forward focussing effect is observed such that the spot size decreases faster than is expected solely from an acceleration effect, indicating an affect from the tip shape. These results have consequences in designing gas field ion sources where etching is used to prepare the emitter.

Negative differential resistance remains an attractive but elusive functionality, so far only finding niche applications. Atom scale entities have shown promising properties, but the viability of device fabrication requires a fuller understanding of electron dynamics than has been possible to date. Using an all-electronic time-resolved scanning tunneling microscopy technique and a Green's function transport model, we study an isolated dangling bond on a hydrogen terminated silicon surface. A robust negative differential resistance feature is identified as a many body phenomenon related to occupation dependent electron capture by a single atomic level. We measure all the time constants involved in this process and present atomically resolved, nanosecond time scale images to simultaneously capture the spatial and temporal variation of the observed feature.

We present a simple, reliable method to fabricate Ir single atom tips (SATs) from polycrystalline wire. An electrochemical etch in CaCl 2 solution is followed by a field assisted reactive gas etch in vacuum at room temperature using oxygen as an etching gas and neon as an imaging gas. Once formed, SATs are cooled to liquid nitrogen temperatures and their underlying structure is examined through evaporation of the apex atoms. Furthermore, a method is developed to repair Ir SATs at liquid nitrogen temperatures when apex atoms evaporate. This method may be used to fabricate Ir SAT ion sources.

This chapter discusses fabrication and experimental evaluation of W(111) single atom tips (SATs) for gas field ion source applications. Firstly, a brief history of field ion microscopy (FIM) will be given since it will be heavily relied on throughout the text. We will discuss ion current generation in FIM and carry that knowledge over to fabricated SATs. Secondly, gas assisted etching and evaporation process will be discussed in detail. It will be shown that nanotip shape, and therefore SAT characteristics, can be controlled and modified to achieve desirable ion beam properties. Lastly, we will evaluate ion beam width as a function of tip voltage and temperature as examples of experimental efforts to better understand gas field ion source performance., Series: NanoScience and Technology

As the ultimate miniaturization of semiconductor devices approaches, it is imperative that the effects of single dopants be clarified. Beyond providing insight into functions and limitations of conventional devices, such information enables identification of new device concepts. Investigating single dopants requires sub-nanometre spatial resolution, making scanning tunnelling microscopy an ideal tool. However, dopant dynamics involve processes occurring at nanosecond timescales, posing a significant challenge to experiment. Here we use time-resolved scanning tunnelling microscopy and spectroscopy to probe and study transport through a dangling bond on silicon before the system relaxes or adjusts to accommodate an applied electric field. Atomically resolved, electronic pump-probe scanning tunnelling microscopy permits unprecedented, quantitative measurement of time-resolved single dopant ionization dynamics. Tunnelling through the surface dangling bond makes measurement of a signal that would otherwise be too weak to detect feasible. Distinct ionization and neutralization rates of a single dopant are measured and the physical process controlling those are identified., Probing individual impurities will become increasingly important as devices shrink towards the nanoscale. Here Rashidi et al., introduce a method based on time-resolved scanning tunnelling spectroscopy of surface dangling bonds to investigate the dynamics of individual dopants in silicon.

The authors present scanning tunneling microscopy images and mass spectra that show that dosing gases at pressures in the range of 10−6 Torr in an ion-pumped ultrahigh vacuum (UHV) chamber results in a measurable concentration of reactive molecular radicals and atomic hydrogen ions being created. One source of radicals is the fragmentation of the dosed molecule, while another is atomic hydrogen that is re-emitted from the ion pump itself. The dosing of noble gases such as helium also results in harmful radicals escaping the ion pump. These radicals are able to create new reactive sites on a hydrogen-terminated Si(100) surface; they show that these new dangling bonds result in extra molecular line growth in a 2,3-dimethyl-1,3-butadiene dosing experiment. These results serve as a cautionary note to experimenters working with ion-pumped UHV systems and surfaces that are sensitive to radicals, such as hydrogen-terminated Si.

Current–voltage characteristics of tunnel contacts to n+-Si(1 0 0) surfaces are modified through covalently bonded molecules. Scanning tunneling microscopy is used to characterize tunnel contacts to clean, hydrogen terminated and styrene terminated surfaces. The clean surface has pinned energy bands by silicon π∗ surface states resulting in an absence of rectification. Passivation with hydrogen removes band pinning and allows minority and majority carrier transport and a 1:10 forward/reverse current rectification ratio (RRF/R). Styrene also removes band pinning however, it introduces a surface dipole that limits the reverse bias to minority carrier transport and an opposite RRF/R of 10:1 is observed.

The formation of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene derived nanostructures on hydrogen-terminated silicon(1 0 0)-2 × 1 was studied using ultra-high vacuum scanning tunneling microscopy and density functional theory modeling. We find that the steric interactions between the methyl groups on 2,3-dimethyl-1,3-butadiene and the surface hydrogen causes 2,3-dimethyl-1,3-butadiene to grow relatively long, linear structures along one side of a dimer row. However, the less sterically encumbered 1,3-butadiene grows short, less orderly, self-terminating structures that involve at least two dimer rows. These results show that steric effects can be used as a parameter in the rational design of organic nanostructures on silicon surfaces.

Highly ordered hydrogen-terminated silicon surfaces are ideal testing grounds for molecular electronics. However, upon formation of these surfaces it is inevitable that some surface sites are not capped by hydrogen. These remaining dangling bonds can interfere with the chemical and electronic properties of nanostructures formed on the silicon surface. In this work, using scanning tunneling microscopy, high resolution electron energy loss spectroscopy and ab initio computational methods, we explore two chemical approaches to refining the hydrogen termination process. We investigate the utility of diimide (N 2 H 2 ) and N , N -diethylhydroxylamine (DEHA) as hydrogen atom sources that have the ability to cap dangling bonds.

The nanoscale structuring of molecules on silicon surfaces is one approach for combining the tuneable properties of chemical species with the functionality of semiconductor materials. In this study, we report on the growth characteristics of trimethylene sulfide (TMS) on p- and n-type H-Si(100)-2 x 1. The nanostructures formed by TMS on either surface are indistinguishable by scanning tunneling microscopy (STM). However, high-resolution electron energy loss spectroscopy (HREELS) and modeling by density functional theory indicate that the molecular attachment mechanism differs with dopant type. Our results show that TMS adds to a surface silicon dangling bond through the formation of a Si-S bond on p-type silicon and through the formation of a Si-C bond on n-type silicon. In both cases, the added TMS undergoes ring opening following covalent bond formation with the surface. The different ring-opened radicals are able to abstract a hydrogen atom from one of two neighboring silicon dimers. The overall reaction produces TMS-derived nanostructures that grow via a square-wave pattern on the neighboring edges of two dimer rows.

Incorporating diversity into structures constructed from the organic modification of silicon surfaces requires the use of molecules that contain multiple substituents of different types. In this work we examine the possible dissociation pathways of diethylhydroxylamine (DEHA, (C(2)H(5))(2)NOH) on the surface of clean silicon(100)-2x1 using cluster and planewave computational methods and high resolution electron energy loss spectroscopy. Our computational results show that DEHA initially forms a strongly-bound complex with the surface via a dative N-Si bond. A low-barrier O-H bond scission then occurs yielding a surface silicon dimer capped by the (C(2)H(5))(2)NO and H fragments. Calculated and measured vibrational spectra support the computed reaction mechanism.

A new technique for the fabrication of highly sensitive qPlus sensor for atomic force microscopy (AFM) is described. Focused ion beam was used to cut then weld onto a bare quartz tuning fork a sharp micro-tip from an electrochemically etched tungsten wire. The resulting qPlus sensor exhibits high resonance frequency and quality factor allowing increased force gradient sensitivity. Its spring constant can be determined precisely which allows accurate quantitative AFM measurements. The sensor is shown to be very stable and could undergo usual UHV tip cleaning including e-beam and field evaporation as well as in-situ STM tip treatment. Preliminary results with STM and AFM atomic resolution imaging at $4.5\,K$ of the silicon $Si(111)-7\times 7$ surface are presented., 5 pages, 3 figures

We report the study of single dangling bonds (DB) on the hydrogen terminated silicon (100) surface using a low temperature scanning tunneling microscope (LT-STM). By investigating samples prepared with different annealing temperatures, we establish the critical role of subsurface arsenic dopants on the DB electronic properties. We show that when the near surface concentration of dopants is depleted as a result of $1250{\deg}C$ flash anneals, a single DB exhibits a sharp conduction step in its I(V) spectroscopy that is not due to a density of states effect but rather corresponds to a DB charge state transition. The voltage position of this transition is perfectly correlated with bias dependent changes in STM images of the DB at different charge states. Density functional theory (DFT) calculations further highlight the role of subsurface dopants on DB properties by showing the influence of the DB-dopant distance on the DB state. We discuss possible theoretical models of electronic transport through the DB that could account for our experimental observations., Comment: 21 pages, 6 figures

Peak assignment is a complex but important task for analyzing the vibration spectra of surface-bound molecules. Here we describe a simple approach for calculating infrared and Raman spectra for surface-bound molecules using a cluster model approach with quantum capping potentials (QCPs). The utility of the approach is demonstrated by comparisons to the measured high resolution electron energy loss spectra for ethylene on clean silicon. By capping the silicon cluster with QCPs we computed spectra that agree very well with the HREEL spectrum, allowing us to easily assign the experimental peaks. QCPs are similar to effective core potentials, can be used with any ab initio technique and most computational chemistry packages, and their use requires no special expertise.

Atomically resolved scanning tunneling microscopy and spectroscopy (STM/STS) have been used to carefully examine the relationship between molecular conductivity and the adsorption state of various organic molecules on silicon surfaces. We show that the particular configuration of styrene and cyclopentene molecules on silicon affects the conductivity of the molecules. Detailed correlation of STM images with point specific current-voltage spectroscopy reveal that observed negative peaks are due to random configuration changes driven by inelastically scattered electrons and not due to tuned alignment of molecule and electrode levels. These random processes, which include molecular rearrangement, desorption, and/or decomposition occur with increasing frequency at larger voltage and current settings.

Electrical transport through molecules has been much studied since it was proposed1 that individual molecules might behave like basic electronic devices, and intriguing single-molecule electronic effects have been demonstrated2, 3. But because transport properties are sensitive to structural variations on the atomic scale4, 5, 6, 7, further progress calls for detailed knowledge of how the functional properties of molecules depend on structural features. The characterization of two-terminal structures has become increasingly robust and reproducible8, 9, 10, 11, 12, and for some systems detailed structural characterization of molecules on electrodes or insulators is available13, 14, 15, 16, 17. Here we present scanning tunnelling microscopy observations and classical electrostatic and quantum mechanical modelling results that show that the electrostatic field emanating from a fixed point charge regulates the conductivity of nearby substrate-bound molecules. We find that the onset of molecular conduction is shifted by changing the charge state of a silicon surface atom, or by varying the spatial relationship between the molecule and that charged centre. Because the shifting results in conductivity changes of substantial magnitude, these effects are easily observed at room temperature.

The ability of 25 density functional theory (DFT) methods to treat Ne2, Ar2, (CH4)2, (C2H4)2 and three conformations of the benzene dimer was studied. Only PW91, HTCH407, and VSXC predict all dimers to be bound. However, VSXC strongly over-binds all of the complexes. The DFTs predict repulsive potential energy surfaces (PES) for parallel benzene at large separations, in contrast to MP2. VSXC, B1B95, BB95, and BB1K, and to a lesser extent OLYP and O3LYP, are sensitive to the size of the integration grid used. When used with standard grids, these methods predict multiple minima on one or more dimer PESs.

A chain reaction initiated at a dangling bond on a H-terminated Si(100)-3×1 surface leads to the creation of contiguous, linear multimolecular assemblies. In contrast to a similar growth process observed on the H−Si(100)-2×1 surface, the linear structures grow in the cross-row direction, rather than parallel to dimer rows. This process is enabled by both an uncommonly high rate of H atom diffusion, specifically in the cross-row direction, and a low barrier to H atom abstraction from dihydride sites. These results demonstrate that anisotropy inherent to the substrate can be imposed upon molecular assemblies formed via this “self-directed” growth process.

The scanning ion microscopy is gaining momentum as it provides several key advantages over scanning electron microscopy: (i) enhanced depth of focus, (ii) improved surface and element sensitivity, (iii) better lateral resolution, and (iv) nanomachining and milling. It uses different ions to achieve these tasks ranging from inert gases like helium and neon for imaging and ion milling. Other gases such as argon, nitrogen, and oxygen have potential for further sputtering and etching. It is therefore crucial that gas field ion sources provide necessary robustness and stability for range of various gases.

Cyclopropyl methyl ketone molecules react at single dangling bonds on an H-terminated Si(100) surface to form a metastable radical species attached via the oxygen atom. While the adsorbate has the capacity to abstract an H atom from an adjacent surface site, in analogy with the previously studied reaction of styrene, it appears not to do so. Rather, the cyclopropyl ring opens, thereby shifting the radical to a position distant from the point of attachment. H-abstraction then occurs from one of a variety of surface sites within range of the radical. The surface dangling bond created by abstraction allows the process to repeat, leading to the creation of a contiguous string of molecules attached to the surface. The assembly process follows a meandering path unlike in the case of styrene where H-abstraction is restricted to adjacent surface sites, resulting in straight multimolecular assemblies. This result hints at an opportunity to gain control over growth direction through judicious addition of constraint...