Coherent single charge transport in molecular-scale silicon nanowires

We report low-temperature electrical transport studies of chemically synthesized, molecular-scale silicon nanowires. Individual nanowires exhibit Coulomb blockade oscillations characteristic of charge addition to a single nanostructure on length scales up to at least 400 nm. Studies also demonstrate coherent charge transport through discrete single particle quantum levels extending across whole devices, and show that the ground-state spin configuration is consistent with the constant interaction model. In addition, depletion of nanowires suggests that phase coherent single-dot characteristics are accessible in the few-charge regime. These results differ from those for nanofabricated planar silicon devices, which show localization on much shorter length scales, and thus suggest potential for molecular-scale silicon nanowires as building blocks for quantum and conventional electronics.     

Direct measurement of individual deep traps in single silicon nanowires

The potential of the metal nanocatalyst to contaminate vapor-liquid-solid (VLS) grown semiconductor nanowires has been a long-standing concern, since the most common catalyst material, Au, is known to induce deep gap states in several semiconductors. Here we use Kelvin probe force microscopy to image individual deep acceptor type trapping centers in single undoped Si nanowires grown with an Au catalyst. The switching between occupied and empty trap states is reversibly controlled by the back-gate potential in a nanowire transistor. The trap energy level, i.e., E(C) - E(T) = 0.65 ± 0.1 eV was extracted and the concentration was estimated to be ～2 × 10(16) cm(-3). The energy and concentration are consistent with traps resulting from the unintentional incorporation of Au atoms during the VLS growth.     

Synthesis of silica nanowires by active oxidation of silicon substrates

Amorphous silica nanowires have been produced by thermal annealing of Si/SiO2/Ni substrate structures at 900 degrees C under an atmosphere of hexamethyldisilazane (HMDS) and hydrogen (H2). The wires have diameter ranging from 35 to 55 nm, which are controlled by the Ni particle size. It is demonstrated that the growth occurs through vapor-liquid-solid mechanisms, and it is proposed that the vapor source is volatile SiO generated from the etching of the Si substrate through active oxidation reactions. The role of the HMDS-H2 atmosphere in promoting such reactions is discussed.     

Unveiling the formation pathway of single crystalline porous silicon nanowires

Porous silicon nanowire is emerging as an interesting material system due to its unique combination of structural, chemical, electronic, and optical properties. To fully understand their formation mechanism is of great importance for controlling the fundamental physical properties and enabling potential applications. Here we present a systematic study to elucidate the mechanism responsible for the formation of porous silicon nanowires in a two-step silver-assisted electroless chemical etching method. It is shown that silicon nanowire arrays with various porosities can be prepared by varying multiple experimental parameters such as the resistivity of the starting silicon wafer, the concentration of oxidant (H(2)O(2)) and the amount of silver catalyst. Our study shows a consistent trend that the porosity increases with the increasing wafer conductivity (dopant concentration) and oxidant (H(2)O(2)) concentration. We further demonstrate that silver ions, formed by the oxidation of silver, can diffuse upwards and renucleate on the sidewalls of nanowires to initiate new etching pathways to produce a porous structure. The elucidation of this fundamental formation mechanism opens a rational pathway to the production of wafer-scale single crystalline porous silicon nanowires with tunable surface areas ranging from 370 to 30 m(2) g(-1) and can enable exciting opportunities in catalysis, energy harvesting, conversion, storage, as well as biomedical imaging and therapy.     

Hidden defects in silicon nanowires

Recent publications have reported the presence of hexagonal phases in Si nanowires. Most of these reports were based on 'odd' diffraction patterns and HRTEM images—'odd' means that these images and diffraction patterns could not be obtained on perfect silicon crystals in the classical diamond cubic structure. We analyze the origin of these 'odd' patterns and images by studying the case of various Si nanowires grown using either Ni or Au as catalysts in combination with P or Al doping. Two models could explain the experimental results: (i) the presence of a hexagonal phase or (ii) the presence of defects that we call 'hidden' defects because they cannot be directly observed in most images. We show that in many cases one direction of observation is not sufficient to distinguish between the two models. Several directions of observations have to be used. Secondly, conventional TEM images, i.e. bright-field two-beam and dark-field images, are of great value in the identification of 'hidden' defects. In addition, slices of nanowires perpendicular to the growth axis can be very useful. In the studied nanowires no hexagonal phase with long range order is found and the 'odd' images and diffraction patterns are mostly due to planar defects causing superposition of different crystal grains. Finally, we show that in Raman experiments the defect-rich NWs can give rise to a Raman peak shifted to 504–511 cm?1 with respect to the Si bulk peak at 520 cm?1, indicating that Raman cannot be used to identify a hexagonal phase.     

Growth of high-density titanium silicide nanowires in a single direction on a silicon surface

Understanding the growth mechanisms of nanowires is essential for their successful implementation in advanced devices applications. In situ ultrahigh-vacuum transmission electron microscopy has been applied to elucidate the interaction mechanisms of titanium disilicide nanowires (TiSi2 NWs) on Si(111) substrate. Two phenomena were observed: merging of the two NWs in the same direction, and collapse of one NW on a competing NW in a different direction when they meet at the ends. On the other hand, as one NW encounters the midsection of the other NW in a different direction, it recedes in favor of bulging of the other NW at the midsection. Since crystallographically the nanowires are favored to grow on Si(110) only in the [1 -1 0] direction, this crucial information has been fruitfully exploited to focus on the growth of a high density of long and high-aspect-ratio Ti silicide NWs parallel to the surface on Si(110) in a single direction. The achievement in growth of high-density NWs in a single direction represents a significant advance in realizing the vast potential for applications of silicide NWs in nanoelectronics devices.     

Multi-parametric growth of silicon nanowires in a single platform by laser-induced localized heat sources

Nanoscale-synthesized materials hold great promise for the realization of future generation devices. In order to fulfil this exceptional promise, new techniques must be developed that will enable the precise layout and assembly of the heterogeneous components into functional 'superblocks'. Direct synthesis of nanostructures via a laser-assisted chemical vapor deposition process is one promising route. In this paper, laser-assisted silicon nanowire growth based on a vapor-liquid-solid (VLS) mechanism is studied. Spatial confinement of the nanowire growth region via focused laser beam illumination provides a convenient way to examine multiple growth parameters (temperature, time, illumination direction, gas species composition, and pressure), thereby elucidating fundamental mechanisms of laser-assisted growth in a single sample configuration. Furthermore, the work demonstrates an advanced method for direct synthesis of nanostructures for the purpose of practical rapid patterning.     

Coexistence of 1D and quasi-0D photoluminescence from single silicon nanowires

Single silicon nanowires (Si-NWs) prepared by electron-beam lithography and reactive-ion etching are investigated by imaging optical spectroscopy under variable temperatures and laser pumping intensities. Spectral images of individual Si-NWs reveal a large variability of photoluminescence (PL) along a single Si-NW. The weaker broad emission band asymmetrically extended to the high-energy side is interpreted to be due to recombination of quasi-free 1D excitons while the brighter localized emission features (with significantly variable peak position, width, and shape) are due to localization of electron-hole pairs in surface protrusions acting like quasi-0D centers or quantum dots (QDs). Correlated PL and scanning electron microscopy images indicate that the efficiently emitting QDs are located at the Si-NW interface with completely oxidized neck of the initial Si wall. Theoretical fitting of the delocalized PL emission band explains its broad asymmetrical band to be due to the Gaussian size distribution of the Si-NW diameter and reveals also the presence of recombination from the Si-NW excited state which can facilitate a fast capture of excitons into QD centers.     

Multicolored vertical silicon nanowires

We demonstrate that vertical silicon nanowires take on a surprising variety of colors covering the entire visible spectrum, in marked contrast to the gray color of bulk silicon. This effect is readily observable by bright-field microscopy, or even to the naked eye. The reflection spectra of the nanowires each show a dip whose position depends on the nanowire radii. We compare the experimental data to the results of finite difference time domain simulations to elucidate the physical mechanisms behind the phenomena we observe. The nanowires are fabricated as arrays, but the vivid colors arise not from scattering or diffractive effects of the array, but from the guided mode properties of the individual nanowires. Each nanowire can thus define its own color, allowing for complex spatial patterning. We anticipate that the color filter effect we demonstrate could be employed in nanoscale image sensor devices.     

Infrared spectra of silicon nanowires

Infrared (IR) spectra of the silicon nanowires (SiNWs) with oxide layer are analyzed by introducing the disorder-induced mechanical coupling between the optically active oxygen asymmetric stretch (AS) and inactive oxygen asymmetric stretch (I-AS) modes in terms of the transverse-optic (TO) and longitudinal-optic (LO) vibrational modes. The shapes of the IR spectra are similar to that of the reported SiO₂, indicating that the SiNWs possess an oxide layer outside. The TO frequencies of coupled AS and I-AS are experimentally observed as peak at approximately 1085 cm^− 1 and its shoulder of 1200 cm^− 1, respectively. The other TO absorption peaks of ∼ 468 cm^− 1, ∼ 480 cm^− 1, and ∼ 808 cm^− 1 are also observed. Furthermore, the intensity of the AS-mode TO band centered at ∼ 1085 cm^− 1 decreases while those of silicon lattice absorption peaks are enhanced with the crystalline quality increased.     

Sample refinement and manipulation of silicon nanowires: A step towards single wire characterization

Refined silicon nanowires have been prepared by a mild etching process and suspended into liquid in order to make them manageable for individual characterization. A transmission electron microscopy (TEM) study has revealed that the etching starts selectively at defect sites on the wires. This implies that the refined wires have many fewer defects than those made of raw materials. Efforts have been made to mount single nanowires onto the desired electrodes by electrophoresis. Compared with the commonly used microactuation method in the field, this is a far more realistic practical use of the wires that has an industrial value.     

Measurement of carrier mobility in silicon nanowires

We report the first direct capacitance measurements of silicon nanowires (SiNWs) and the consequent determination of field carrier mobilities in undoped-channel SiNW field-effect transistors (FETs) at room temperature. We employ a two-FET method for accurate extraction of the intrinsic channel resistance and intrinsic channel capacitance of the SiNWs. The devices used in this study were fabricated using a top-down method to create SiNW FETs with up to 1000 wires in parallel for increasing the raw capacitance while maintaining excellent control on device dimensions and series resistance. We found that, compared with the universal mobility curves for bulk silicon, the electron and hole mobilities in nanowires are comparable to those of the surface orientation that offers a lower mobility.     

Size-scaling in optical trapping of silicon nanowires

We investigate size-scaling in optical trapping of ultrathin silicon nanowires showing how length regulates their Brownian dynamics, optical forces, and torques. Force and torque constants are measured on nanowires of different lengths through correlation function analysis of their tracking signals. Results are compared with a full electromagnetic theory of optical trapping developed in the transition matrix framework, finding good agreement.     

DNA hybridization on silicon nanowires

Nanowire-based detection strategies provide promising new routes to bioanalysis and indeed are attractive to conventional systems because of their small size, high surface-to-volume ratios, electronic, and optical properties. A sequence-specific detection of single-stranded oligonucleotides using silicon nanowires (SiNWs) is demonstrated. The surface of the SiNWs is functionalized with densely packed organic monolayer via hydrosilylation for covalent attachment. Subsequently, deoxyribonucleic acid (DNA) is immobilized to recognize the complementary target DNA. The biomolecular recognition properties of the nanowires are tested via hybridization with ^γP³² tagged complementary and non-complementary DNA oligonucleotides, showing good selectivity and reversibility. No significant non-specific binding to the incorrect sequences is observed. X-ray photoelectron spectroscopy, fluorescence imaging, and nanodrop techniques are used to characterize the modified SiNWs and covalent attachment with DNA. The results show that SiNWs are excellent substrates for the absorption, stabilization and detection of DNA sequences and could be used for DNA microarrays and micro fabricated SiNWs DNA sensors.     

Silicidation of silicon nanowires by platinum

The solid-state reaction between platinum and silicon nanowires grown by the vapor-liquid-solid technique was studied. The reaction product PtSi is an attractive candidate for contacts to p-type silicon nanowires due to the low barrier height of PtSi contacts to p-type Si in the planar geometry, and the formation of PtSi was the motivation for our study. Silicidation was carried out by annealing Pt on Si nanowires from 250 to 700 degrees C, and the reaction products were characterized by transmission electron microscopy. Strikingly different morphologies of the reacted nanowires were observed depending on the annealing temperature, platinum film thickness, silicon nanowire diameter, and level of unintentional oxygen contamination in the annealing furnace. Conversion to PtSi was successfully realized by annealing above 400 degrees C in purified N2 gas. A uniform morphology was achieved for nanowires with an appropriate combination of Si nanowire diameter and Pt film thickness to form PtSi without excess Pt or Si. Similar to the planar silicidation process, oxygen affects the nanowire silicidation process greatly.     

Ion beam doping of silicon nanowires

We demonstrate n- and p-type field-effect transistors based on Si nanowires (SiNWs) implanted with P and B at fluences as high as 10(15) cm (-2). Contrary to what would happen in bulk Si for similar fluences, in SiNWs this only induces a limited amount of amorphization and structural disorder, as shown by electrical transport and Raman measurements. We demonstrate that a fully crystalline structure can be recovered by thermal annealing at 800 degrees C. For not-annealed, as-implanted NWs, we correlate the onset of amorphization with an increase of phonon confinement in the NW core. This is ion-dependent and detectable for P-implantation only. Hysteresis is observed following both P and B implantation.     

Phase transition and compressibility in silicon nanowires

Silicon nanowires (Si NWs), one-dimensional single crystalline, have recently drawn extensive attention, thanks to their robust applications in electrical and optical devices as well as in the strengthening of diamond/SiC superhard composites. Here, we conducted high-pressure synchrotron diffraction experiments in a diamond anvil cell to study phase transitions and compressibility of Si NWs. Our results revealed that the onset pressure for the Si I-II transformation in Si NWs is approximately 2.0 GPa lower than previously determined values for bulk Si, a trend that is consistent with the analysis of misfit in strain energy. The bulk modulus of Si-I NWs derived from the pressure-volume measurements is 123 GPa, which is comparable to that of Si-V NWs but 25% larger than the reported values for bulk silicon. The reduced compressibility in Si NWs indicates that the unique wire-like structure in nanoscale plays vital roles in the elastic behavior of condensed matter.     

Kinetically-induced hexagonality in chemically grown silicon nanowires

Various silicon crystal structures with different atomic arrangements from that of diamond have been observed in chemically synthesized nanowires. The structures are typified by mixed stacking mismatches of closely packed Si dimers. Instead of viewing them as defects, we define the concept of hexagonality and describe these structures as Si polymorphs. The small transverse dimensions of a nanowire make this approach meaningful. Unique among the polymorphs are cubic symmetry diamond and hexagonal symmetry wurtzite structures. Electron diffraction studies conducted with Au as an internal reference unambiguously confirm the existence of the hexagonal symmetry Si nanowires. Cohesive energy calculations suggest that the wurtzite polymorph is the least stable and the diamond polymorph is the most stable. Cohesive energies of intermediate polymorphs follow a linear trend with respect to their structural hexagonality. We identify the driving force in the polymorph formations as the growth kinetics. Fast longitudinal elongation during the growth freezes stacking mismatches and thus leads to a variety of Si polymorphs. The results are expected to shed new light on the importance of growth kinetics in nanomaterial syntheses and may open up ways to produce structures that are uncommon in bulk materials. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. This article is published with open access at Springerlink.com