Silicon Nanogap Research Articles

Gap distance dependence on field emission at the nanogap between silicon cleavage surfaces

In this study, we developed a model of field emission at a vacuum nanogap composed of semiconductor electrodes and conducted numerical calculations to evaluate its field emission characteristics. The current at the silicon nanogap was also measured, and the results were compared with the calculated results. Large-area vacuum nanogaps are expected to be used for highly efficient room-temperature thermionic power generation owing to their good thermal insulation, and their electrical conduction of tunneling current is sufficient for practical use. Because vacuum nanogaps should be parallel and smooth with a large area, we proposed and examined the use of cleavage to fabricate nanogaps in single-crystal materials. A nanogap with a nominal area of 26.5 μm2 was fabricated in a microelectromechanical device to evaluate its properties, and the model was used for the analysis. The calculated current–voltage characteristics of the semiconductor nanogap of both n-type and p-type silicon were affected by the bandgap and the presence of carriers, i.e., electrons and holes. Moreover, the Fowler–Nordheim plot was confirmed to be valid for semiconductor nanogaps based on the calculated results. We proposed an effective work function that compensates for the discrepancy between semiconductor nanogaps and the Fowler–Nordheim theory and enables a more accurate analysis. Field-emission currents flowing through a 1.1 nm nanogap were measured and matched with the calculated results by adjusting the field enhancement factor and emission area. The model developed and confirmed experimentally in this work will be helpful in examining field emissions at semiconductor nanogaps.

Local Cracking‐Induced Scalable Flexible Silicon Nanogaps for Dynamically Tunable Surface Enhanced Raman Scattering Substrates

AbstractSurface enhanced Raman scattering (SERS), as a promising and convenient analytical tool for molecule sensing, has turned out to be one of the most important applications of plasmonic nanostructures. However, its large‐area production, controllability, and adjustability remain significant challenges because of the weak control over the fabrication and spatial arrangement. Herein, a silicon nanogap array‐based flexible plasmonic substrate is developed, with capabilities of large‐area production, controllable arrangement, and width of nanogaps, by utilizing the standard microfabrication platform, and the fabricated flexible plasmonic substrate is further explored for dynamically tunable SERS for molecular sensing. After experimentally and theoretically optimizing the geometric designs of precursors, a 90 × 90 silicon nanogap array (over 1 cm × 1 cm) with a yield of about 99.74% is achieved. SERS is realized by depositing Au films onto the silicon nanogap. Dynamically tunable SERS is demonstrated by the thermally induced local stretch or contraction of silicon nanogaps, where the key parameters including Raman enhancement and sensitivity of the molecular sensor can be conveniently adjusted. These presented results significantly expand the scope of engineering opinions for flexible plasmonic nanostructures, which have many envisioned applications especially for environmental monitoring and biochemical detection.

Nanogaps formation and characterization via chemical and oxidation methods

Different materials were used to optimize the desired nano-size with smooth process and faster fabrication. Gold, polysilicon and silicon were used to apply this report (experimental). SOI and Si wafers have used as a substrate and one chrome mask to build up the nanogap devices using size reduction technique. Two chrome masks have used to fabricate the proposed pattern. Electrical characterization was applied to setup the fabricated devices with different materials. Conductivity and resistivity were measured to characterize the nanogap structure with gold, polysilicon and silicon as electrodes. However, gold nanogap has recorded an increment in the conductivity, and the silicon nanogap structures have recorded an increment in the resistivity comparing with the other used materials.

DNA hybridization detection using less than 10-nm gap silicon nanogap structure

This report described less than 10-nm gap silicon-based electrode sensor for the discriminating detection of complementary, non-complementary and single nucleotide mismatched DNA targets. Discriminating detection of nucleic acid sequences is a key step in biodiagnostics, drug discovery, gene expression profiling, environmental monitoring and forensic studies. The method is based on the measurement of capacitance, conductance and permittivity profiles of the nanogap electrodes upon target oligonucleotide hybridization using a simple dielectric analyzer. The nanogap surface was at first modified with a self-assembled layer of amino functionalities followed by non-covalent adsorption of gold nanoparticles to create an attachment surface for the thiol-modified DNA probe. The capacitance, conductance and permittivity profiles of the biosensor clearly differentiated complementary, non-complementary and single mismatch target hybridization without the need of any labeling steps, enzymatic treatment or metalization chemistry. The detection limit and range of the silicon nanogap biosensor were 3nmol/L and 3–15nmol/L of target DNA.

Gating electrical transport through DNA molecules that bridge between silicon nanogaps

DNA electronic devices were prepared on silicon-based three-terminal electrodes. Both ends of DNA molecules (400 bp long, mixed sequences) were bridged via chemical bonds between the source-drain nanogap (120 nm) electrodes. S-Shaped I-V curves were obtained and the electric current can be modulated by the gate voltage. The DNA molecules act as semiconducting p-type nanowires in the three-terminal device.

Synthesis of Covalently Linked Molecular Bridges between Silicon Electrodes in CMOS‐Based Arrays of Vertical Si/SiO2/Si Nanogaps

Silicon nanogaps were bridged in situ by grafting 4-ethynylbenzaldehyde to activate the electrodes and coupling 2,6-diaminoanthra-9,10-quinone to link the coatings. The bridged structures exhibit currents of 11–14 nA at 1 V. The process is reversed by soaking in acidified solution, which causes the current to diminish.

Fabrication and Characterization of 50 nm Silicon Nano-Gap Structures

A simple method for the fabrication of nano-gaps less than 50 nm by using conventional photolithography combined with patterned-size reduction techniques is presented. Silicon material is used to fabricate the nano-gap structure and gold is used for the electrode. Two chrome masks are proposed to complete this work, the first mask for the nano-gap pattern and a second mask for the electrode. The method is based on the control of the coefficients (temperature and time) with an improved pattern size resolution by thermal oxidation. With this technique, there are no principal limitations to fabricating nanostructures with different layouts down to several nanometers in dimension. In this work, the proposed method is experimentally demonstrated by preparing the nano-gaps on a Si–SiO2 substrate down to dimensions of 50 nm. The optical characterization that is applied to check the nano-gap structure is by using the scanning electron microscope (SEM).

Molecular Bridging of Silicon Nanogaps

The highly doped electrodes of a vertical silicon nanogap device have been bridged by a 5.85 nm long molecular wire, which was synthesized in situ by grafting 4-ethynylbenzaldehyde via C-Si links to the top and bottom electrodes and thereafter by coupling an amino-terminated fluorene unit to the aldehyde groups of the activated electrode surfaces. The number of bridging molecules is constrained by relying on surface roughness to match the 5.85 nm length with an electrode gap that is nominally 1 nm wider and may be controlled by varying the reaction time: the device current increases from ≤1 pA at 1 V following the initial grafting step to 10-100 nA at 1 V when reacted for 5-15 min with the amino-terminated linker and 10 μA when reacted for 16-53 h. It is the first time that both ends of a molecular wire have been directly grafted to silicon electrodes, and these molecule-induced changes are reversible. The bridges detach when the device is rinsed with dilute acid solution, which breaks the imine links of the in situ formed wire and causes the current to revert to the subpicoampere leakage value of the 4-ethynylbenzaldehyde-grafted nanogap structure.

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.

Reduction of current instabilities in silicon nanogaps

Silicon nanogaps are electrodes for connecting nanoscale elements to silicon technology. Two silicon electrodes are separated by a thin silicon dioxide layer, which is removed by selective etching to create a nanometer sized accessible gap. Parasitic gap currents with large instabilities occur after the etching. These are compared to the instabilities in ordinary MOS (metal oxide silicon) devices after soft breakdown. Clear similarities are seen when subjecting the devices to constant voltage stress, storing them and annealing them. We discuss a percolation-based model for the instable current, in which the percolation clusters are defects in the oxide surface created during the etching, or possibly contaminating residuals from the same process.

Low frequency noise in silicon nanogaps

Silicon nanogaps are contact structures for connecting organic molecules. An insulating layer is removed by etching, and this dramatically increases the current levels and the noise, which closely resembles a 1∕f law and scales with the square of the current. After etching, the noise level at 30Hz and 10nA is in the order of 10−21A2∕Hz, which is more than two orders of magnitude larger than before etching. We model the noisy behavior by several percolation paths in parallel at the etched surface between the electrodes, and compare it with soft breakdown in thin oxide.

Electrical properties of Si–SiO2–Si nanogaps

The chances of attaching organic molecules to silicon surfaces can be considerablyenhanced if a robust nanogap structure with silicon electrodes can be used to connect themolecules. We describe the electrical properties of such an electrode structure, with aseparation of the silicon surfaces in the 3–7 nm range. These silicon nanogaps aremanufactured by partly removing the silicon dioxide insulator from a silicon–oxide–siliconmaterial stack, by using a selective oxide etchant. After the activation of the gap (theetching), current instabilities appear, which are comparable to the properties of thin oxidesafter soft breakdown. Applying a constant voltage can reduce these current instabilities.We also address the issue of surface leakage currents for these nanogap structures.

Mass-fabricated one-dimensional silicon nanogaps for hybrid organic/nanoparticlearrays

Optical lithography based on microfabrication techniques was employed to fabricateone-dimensional nanogaps with micrometre edge lengths in silicon. These one-dimensionalnanogaps served as a platform on which organic/nanoparticle films were assembled.Characterization of the gaps was performed with high-resolution TEM, SEM, andelectrical measurements. Novel self-assembling attachment chemistry, based onthe interaction of silicon with a diazonium salt, was used to iteratively build amulti-layer nanoparticle film across a 7 nm gap. By using nanoparticles capped with aneasily displaced ligand, a variable conductive path was created across the 1Dnanogap. Electrical measurements of the gap showed a dramatic change in theI(V) characteristics after assembly of the nanoparticle film.

Fabrication of a Separation by Implantation of Oxygen Silicon Nano-Gap Using Thin Film Stress

We have fabricated and controlled a novel nanometer scale silicon gap in a separation by implantation of oxygen (SIMOX) structure using the thin film stress generated during high temperature annealing and cooling, and then elucidated the formation mechanism and crystal orientation dependency of the gaps. The technique is very simple and controllable in shaping a wedge type nano-structure with a several hundred-nanometer gap. The fabricated structure has a 190 and 250 nm narrow gap with 2 µm and 11 µm side etching of the buried SiO2, respectively.