Oxidation Scanning Probe Lithography Research Articles

Nanopatterning of thin amorphous vanadium oxide films by oxidation scanning probe lithography

Oxidation scanning probe lithography is one of the most promising techniques for nanostructuring. Vanadium oxides constitute a class of functionally rich materials that are promising for various practical applications and have unique physical properties. Currently, the formation of nanocrystals and nanostructures with well-defined dimensions is a challenging task. This study presents a detailed investigation of the processes involved in oxidation scanning probe lithography on the surface of thin amorphous vanadium oxide (VOx) films. It was shown that the oxidizable regions of the film transform into a water-soluble vanadium pentoxide (V2O5) in accordance with classical redox reactions. The oxidation kinetics was shown to be consistent with the Cabrera-Mott model. Dissolving the oxidized regions in water resulted in the formation of nanohole arrays with defined sizes in VOx films. Nanostructures with a depth of less than 0.3 nm and lateral dimensions of less than 50 nm were obtained at relative humidity of about 10 %. It was demonstrated that VOx films about 10 nm thick can be completely oxidized in local areas, enabling the formation of isolated nanostructures. The considered method of nanostructuring VOx is promising for the formation of novel photonic and nanoelectronic devices.

Molecular Recognition by Silicon Nanowire Field-Effect Transistor and Single-Molecule Force Spectroscopy.

Silicon nanowire (SiNW) field-effect transistors (FETs) have been developed as very sensitive and label-free biomolecular sensors. The detection principle operating in a SiNW biosensor is indirect. The biomolecules are detected by measuring the changes in the current through the transistor. Those changes are produced by the electrical field created by the biomolecule. Here, we have combined nanolithography, chemical functionalization, electrical measurements and molecular recognition methods to correlate the current measured by the SiNW transistor with the presence of specific molecular recognition events on the surface of the SiNW. Oxidation scanning probe lithography (o-SPL) was applied to fabricate sub-12 nm SiNW field-effect transistors. The devices were applied to detect very small concentrations of proteins (500 pM). Atomic force microscopy (AFM) single-molecule force spectroscopy (SMFS) experiments allowed the identification of the protein adsorption sites on the surface of the nanowire. We detected specific interactions between the biotin-functionalized AFM tip and individual avidin molecules adsorbed to the SiNW. The measurements confirmed that electrical current changes measured by the device were associated with the deposition of avidin molecules.

Facile fabrication for a stable interface in 2D materials/graphene van der Waals heterostructure

In order to fully utilize the excellent electrical properties of graphene as an electrode, it is essential to preserve the nature of pristine graphene. However, structural defects or polymer residues during the conventional fabrication steps are inevitable, severely limiting device performance. To overcome these issues, we used a seamless lateral graphene–graphene oxide (GO)-graphene layer fabricated by oxidation scanning probe lithography as electrodes of the MoS2 field-effect transistor. We demonstrated residue-free and flawless graphene surfaces and furthermore GO interlayer between the MoS2 and gate dielectric reduces interface roughness and screens interface traps, leading to improved electron injection and carrier mobility.

Sub-10 nm patterning of few-layer MoS2 and MoSe2 nanolectronic devices by oxidation scanning probe lithography

The properties of 2D materials devices are very sensitive to the physical, chemical and structural interactions that might happen during processing. Low-invasive patterning methods are required to fabricate devices at the nanoscale. Here we developed a process that combines oxidation scanning probe lithography (o-SPL) and oxygen plasma to fabricate nanoribbon field-effect transistors and nano-constrictions on few-layer MoS2 and MoSe2. The oxygen plasma has a double role in this process. First, it forms a thin, uniform oxide layer on top of the flake surface to enable o-SPL nanopatterning with full control of shape and size. Second, the oxide layer thins down the flake. Both plasma-based and o-SPL oxides are soluble in deionized H2O, which enabled etching and the definition of electrically isolated nano-constrictions and nanoribbons. The accuracy and robustness of the process was applied to pattern sub-10 nm wide constrictions and nanoribbon transistors.

Quantitative phase-mode electrostatic force microscopy on silicon oxide nanostructures.

Phase-mode electrostatic force microscopy (EFM-Phase) is a viable technique to image surface electrostatic potential of silicon oxide stripes fabricated by oxidation scanning probe lithography, exhibiting an inhomogeneous distribution of localized charges trapped within the stripes during the electrochemical reaction. We show here that these nanopatterns are useful benchmark samples for assessing the spatial/voltage resolution of EFM-phase. To quantitatively extract the relevant observables, we developed and applied an analytical model of the electrostatic interactions in which the tip and the surface are modelled in a prolate spheroidal coordinates system, fitting accurately experimental data. A lateral resolution of ∼60 nm, which is comparable to the lateral resolution of EFM experiments reported in the literature, and a charge resolution of ∼20 electrons are achieved. This electrostatic analysis evidences the presence of a bimodal population of trapped charges in the nanopatterned stripes.

Roles of structural and chemical defects in graphene on quenching of nearby fluorophores

Oxidation scanning probe lithography was used to systematically fabricate graphene/defective graphene ribbon arrays on a graphene sheet. The tribological, structural, and chemical natures of the defects created by o-SPL were characterized using lateral force microscopy (LFM), micro-Raman (μ-RS) and micro-X-ray spectroscopy (μ-XPS), respectively. While LFM revealed all fabricated patterns exhibited increase in friction, μ-RS and μ-XPS analysis showed that a transition of defect type from structural to chemical defects occurred in the defective graphene ribbons at a threshold voltage (8.5 V). Subsequently, micro-photoluminescence (μ-PL) quenching spectroscopy was measured for a thin layer of polystyrene containing isolated fluorophores (MEH-PPV) spin cast on the ribbon patterns. Whilst the quenching efficiency was suppressed more significantly as defect concentration increased, for pure structural defects, the μ-PL lifetime varied linearly with defect concentration and then saturated to a constant as chemical defects dominated. Among all oxygen related functional groups, the presence of CO bonds led to greatly reduced quenching efficiency while simultaneously reducing the strong blue spectral shift seen for pure graphene with the spectrum at higher CO bond concentration approaching to pristine MEH-PPV. This work demonstrated that local chemical and structural composition in 2D quencher strongly affects the quenching efficiency and dynamics.

Revisiting Oxidation Scanning Probe Lithography of Graphene: Balance of Water Condensation Energy and Electrostatic Energy

We present a study on oxidation scanning probe lithography (o-SPL) of chemical vapor deposition grown graphene conducted under a pulsed voltage bias (Vbias) with millisecond rise time. It is found ...

Direct Patterning of p-Type-Doped Few-layer WSe2 Nanoelectronic Devices by Oxidation Scanning Probe Lithography.

Direct, robust, and high-resolution patterning methods are needed to downscale the lateral size of two-dimensional materials to observe new properties and optimize the overall processing of these materials. In this work, we report a fabrication process where the initial microchannel of a few-layer WSe2 field-effect transistor is treated by oxygen plasma to form a self-limited oxide layer on top of the flake. This thin oxide layer has a double role here. First, it induces the so-called p-doping effect in the device. Second, it enables the fabrication of oxide nanoribbons with controlled width and depth by oxidation scanning probe lithography (o-SPL). After the removal of the oxides by deionized H2O etching, a nanoribbon-based field-effect transistor is produced. Oxidation SPL is a direct writing technique that minimizes the use of resists and lithographic steps. We have applied this process to fabricate a 5 nm thick WSe2 field-effect transistor, where the channel consists in an array of 5 parallel 350 nm half-pitch nanoribbons. The electrical measurements show that the device presents an improved conduction level compared to the starting thin-layer transistor and a positive threshold voltage shift associated to the p-doping treatment. The method enables to pattern devices with sub-50 nm feature sizes. We have patterned an array of 10 oxide nanowires with 36 nm half-pitch by oxidation SPL.

Sub-30 nm patterning of molecular resists based on crosslinking through tip based oxidation

Oxidation Scanning Probe Lithography (o-SPL) is an established method employed for device patterning at the nanometer scale. It represents a feasible and inexpensive alternative to standard lithographic techniques such as electron beam lithography (EBL) and nanoimprint lithography (NIL). In this work we applied non-contact o-SPL to an engineered class of molecular resists in order to obtain crosslinking by electrochemical driven oxidation. By patterning and developing various resist formulas we were able to obtain a reliable negative tone resist behavior based on local oxidation. Under optimal conditions, directly written patterns can routinely reach sub-30 nm lateral resolution, while the final developed features result wider, approaching 50 nm width.

Chemical and structural analysis of sub-20 nm graphene patterns generated by scanning probe lithography

Sub-20 nm patterns have been fabricated by using oxidation scanning probe lithography on epitaxial graphene. The structural and chemical properties of these nanopatterns have been characterized by high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and electron energy loss spectroscopy. The electron microscopy images reveal that the nanolithography process modifies the graphene monolayer and a thin region of the SiC substrate (1 nm thick). Spatially-resolved electron spectroscopies show that the nanopatterns are made of graphene oxide. The combination of spatially-resolved structural and chemical analysis of graphene nanopatterns will enable the development of high-performance graphene devices.

Local anodic oxidation of solid GeO films: The nanopatterning possibilities

Metastable germanium monoxide (GeO) thin-insulating films have been investigated as a new promising material for oxidation scanning probe lithography. Amorphous GeO films were deposited onto cool Si substrates by thermal evaporation of GeO2<Ge-NCs> heterostructure in vacuum. Properties of GeO films were studied by means of IR spectroscopy, Raman spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy. The nanopatterning of GeO films included three stages. First, AFM-induced local anodic oxidation of GeO layer was used to obtain GeO2 nanowires on Si substrate. After local anodic oxidation in high voltage (≥9 V) regime at 80% relative humidity, the cross-section profile of fabricated GeO2 protrusions contained anomalously high double peaks on a broad base (“two-story shape”). Then, thermal annealing was employed to decompose the GeO film into a GeO2 matrix and Ge nanoclusters. Third, after selective etching of GeO2 from the decomposed GeO film, trenches remained in the porous Ge layer instead of GeO2 nanowires. This may be a potentially useful lithographic approach for fabricating nanoscale structures.

Sub-20 nm patterning of thin layer WSe2 by scanning probe lithography

The electronic properties of thin layer transition metal dichalcogenides have raised considerable interest in the fabrication of advanced field-effect transistors and ultrasensitive sensors. Downscaling those devices to the nanoscale depends on the development of cost-effective and robust alternative nanolithographies. Here we demonstrate the direct, resist-less and reproducible nanopatterning of tungsten diselenide thin layers. By using oxidation scanning probe lithography (o-SPL) we have generated arrays of dots with a width of 13 nm and periodicity of 40 nm. We have also patterned a point contact of 35 nm and a nanoscale field-effect transistor. The direct and resistless fabrication of WSe2 nanoscale devices by oxidation scanning probe lithography opens a straightforward and reliable method for processing transition metal dichalcogenides materials.

Direct fabrication of thin layer MoS2 field-effect nanoscale transistors by oxidation scanning probe lithography

Thin layer MoS2-based field effect transistors (FET) are emerging candidates to fabricate very fast and sensitive devices. Here, we demonstrate a method to fabricate very narrow transistor channel widths on a single layer MoS2 flake connected to gold electrodes. Oxidation scanning probe lithography is applied to pattern insulating barriers on the flake. The process narrows the electron path to about 200 nm. The output and transfer characteristics of the fabricated FET show a behavior that is consistent with the minimum channel width of the device. The method relies on the direct and local chemical modification of MoS2. The straightforward character and the lack of specific requirements envisage the controlled patterning of sub-100 nm electron channels in MoS2 FETs.

Fabrication of sub-12 nm thick silicon nanowires by processing scanning probe lithography masks

Silicon nanowires are key elements to fabricate very sensitive mechanical and electronic devices. We provide a method to fabricate sub-12 nm silicon nanowires in thickness by combining oxidation scanning probe lithography and anisotropic dry etching. Extremely thin oxide masks (0.3–1.1 nm) are transferred into nanowires of 2–12 nm in thickness. The width ratio between the mask and the silicon nanowire is close to one which implies that the nanowire width is controlled by the feature size of the nanolithography. This method enables the fabrication of very small single silicon nanowires with cross-sections below 100 nm2. Those values are the smallest obtained with a top-down lithography method.

Electrical characteristics of silicon nanowire transistors fabricated by scanning probe and electron beam lithographies

Silicon nanowire (SiNW) field-effect transistors have been fabricated by oxidation scanning probes and electron beam lithographies. The analysis and comparison of the electron mobility and subthreshold swing shows that the device performance is not affected by the top-down fabrication method. The two methods produce silicon nanowire transistors with similar electrical features, although oxidation scanning probe lithography generates nanowires with smaller channel widths. The values of the electron mobility and the subthreshold swing, 200 cm2 V−1 s−1 and 500 mV dec−1, respectively, are similar to those obtained from bottom-up methods. The compatibility of top-down methods with CMOS (complementary metal–oxide–semiconductor) procedures, the good electrical properties of the nanowire devices and the potential for making sub-10 nanowires, in particular by using oxidation scanning probe lithography, make those methods attractive for device fabrication.