Nanocarbon-scanning probe microscopy synergy: fundamental aspects to nanoscale devices

Exploring the Surface Oxidation and Environmental Instability of 2H-/1T'-MoTe2 Using Field Emission-Based Scanning Probe Lithography

An unconventional approach for the resistless nanopatterning 2H- and 1T'-MoTe2 by means of scanning probe lithography is presented. A Fowler-Nordheim tunneling current of low energetic electrons (E = 30-60 eV) emitted from the tip of an atomic force microscopy (AFM) cantilever is utilized to induce a nanoscale oxidation on a MoTe2 nanosheet surface under ambient conditions. Due to the water solubility of the generated oxide, a direct pattern transfer into the MoTe2 surface can be achieved by a simple immersion of the sample in deionized water. The tip-grown oxide is characterized using Auger electron and Raman spectroscopy, revealing it consists of amorphous MoO3 /MoOx as well as TeO2 /TeOx . With the presented technology in combination with subsequent AFM imaging it is possible to demonstrate a strong anisotropic sensitivity of 1T'-/(Td )-MoTe2 to aqueous environments. Finally the discussed approach is used to structure a nanoribbon field effect transistor out of a few-layer 2H-MoTe2 nanosheet.

Toward Single-Atomic-Layer Lithography on Highly Oriented Pyrolytic Graphite Surfaces Using AFM-Based Electrochemical Etching

Atomic force microscopy (AFM)-based electrochemical etching of a highly oriented pyrolytic graphite (HOPG) surface is studied toward the single-atomic-layer lithography of intricate patterns. Electrochemical etching is performed in the water meniscus formed between the AFM tip apex and HOPG surface due to a capillary effect under controlled high relative humidity (~ 75%) at otherwise ambient conditions. The conditions to etch nano-holes, nano-lines, and other intricate patterns are investigated. The electrochemical reactions of HOPG etching should not generate debris due to the conversion of graphite to gaseous CO and CO₂ based on etching reactions. However, debris is observed on the etched HOPG surface, and incomplete gasification of carbon occurs during the etching process, resulting in the generation of solid intermediates. Moreover, the applied potential is of critical importance for precise etching, and the precision is also significantly influenced by the AFM tip wear. This study shows that the AFM-based electrochemical etching has the potential to remove the material in a single-atomic-layer precision. This result is likely because the etching process is based on anodic dissolution, resulting in the material removal atom by atom.

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.

Controlling the macroscopic electrical properties of reduced graphene oxide by nanoscale writing of electronic channels

The allure of all-carbon electronics stems from the spread of its physical properties across all its allotropes. The scheme also harbours unique challenges, such as tunability of band gap, variability of doping and defect control. Here, we explore the technique of scanning probe tip-induced nanoscale reduction of graphene oxide (GO), which nucleates conducting, [Formula: see text] rich graphitic regions on the insulating GO background. The flexibility of direct writing is supplemented with control over the degree of reduction and tunability of band gap through macroscopic control parameters. The fabricated reduced GO channels and ensuing devices are investigated via spectroscopy and temperature and bias-dependent electrical transport and correlated with spatially resolved electronic properties, using surface potentiometry. The presence of carrier localization effects, induced by the phase-separated [Formula: see text] domains, and large local electric field fluctuations are reflected in the non-linear transport across the channels. Together, the results indicate a complex transport phenomenon, which may be variously dominated by tunnelling or variable range hopping or activated depending on the electronic state of the material.

Micro-Pattern of Graphene Oxide Films Using Metal Bonding

Recently, graphene has been explored in several research areas according to its outstanding combination of mechanical and electrical features. The ability to fabricate micro-patterns of graphene facilitates its integration in emerging technologies such as flexible electronics. This work reports a novel micro-pattern approach of graphene oxide (GO) film on a polymer substrate using metal bonding. It is shown that adding ethanol to the GO aqueous dispersion enhances substantially the uniformity of GO thin film deposition, which is a great asset for mass production. On the other hand, the presence of ethanol in the GO solution hinders the fabrication of patterned GO films using the standard lift-off process. To overcome this, the fabrication process provided in this work takes advantage of the chemical adhesion between the GO or reduced GO (rGO) and metal films. It is proved that the adhesion between the metal layer and GO or rGO is stronger than the adhesion between the latter and the polymer substrate (i.e., cyclic olefin copolymer used in this work). This causes the removal of the GO layer underneath the metal film during the lift-off process, leaving behind the desired GO or rGO micro-patterns. The feasibility and suitability of the proposed pattern technique is confirmed by fabricating the patterned electrodes inside a microfluidic device to manipulate living cells using dielectrophoresis. This work adds great value to micro-pattern GO and rGO thin films and has immense potential to achieve high yield production in emerging applications.

Electrode-Free Anodic Oxidation Nanolithography of Low-Dimensional Materials

Scanning probe lithography based on local anodic oxidation (LAO) provides a robust and general nanolithography tool for a wide range of applications. Its practical use, however, has been strongly hampered due to the requirement of a prefabricated microelectrode to conduct the driving electrical current. Here we report a novel electrode-free LAO technique, which enables in situ patterning of as-prepared low-dimensional materials and heterostructures with great flexibility and high precision. Unlike conventional LAO driven by a direct current, the electrode-free LAO is driven by a high-frequency (>10 kHz) alternating current applied through capacitive coupling, which eliminates the need of a contacting electrode and can be used even for tailoring insulating materials. Using this technique, we demonstrated flexible nanolithography of graphene, hexagonal boron nitride, and carbon nanotubes on insulating substrates with ∼10-nanometer precision. In addition, the electrode-free LAO exhibits high etching quality without oxide residues left. Such an in situ and electrode-free nanolithography with high etching quality opens up new opportunities for fabricating ultraclean nanoscale devices and heterostructures with great flexibility.

Sub-10 Nanometer Feature Size in Silicon Using Thermal Scanning Probe Lithography

High-resolution lithography often involves thin resist layers which pose a challenge for pattern characterization. Direct evidence that the pattern was well-defined and can be used for device fabrication is provided if a successful pattern transfer is demonstrated. In the case of thermal scanning probe lithography (t-SPL), highest resolutions are achieved for shallow patterns. In this work, we study the transfer reliability and the achievable resolution as a function of applied temperature and force. Pattern transfer was reliable if a pattern depth of more than 3 nm was reached and the walls between the patterned lines were slightly elevated. Using this geometry as a benchmark, we studied the formation of 10-20 nm half-pitch dense lines as a function of the applied force and temperature. We found that the best pattern geometry is obtained at a heater temperature of ∼600 °C, which is below or close to the transition from mechanical indentation to thermal evaporation. At this temperature, there still is considerable plastic deformation of the resist, which leads to a reduction of the pattern depth at tight pitch and therefore limits the achievable resolution. By optimizing patterning conditions, we achieved 11 nm half-pitch dense lines in the HM8006 transfer layer and 14 nm half-pitch dense lines and L-lines in silicon. For the 14 nm half-pitch lines in silicon, we measured a line edge roughness of 2.6 nm (3σ) and a feature size of the patterned walls of 7 nm.

Advanced oxidation scanning probe lithography

Force microscopy enables a variety of approaches to manipulate and/or modify surfaces. Few of those methods have evolved into advanced probe-based lithographies. Oxidation scanning probe lithography (o-SPL) is the only lithography that enables the direct and resist-less nanoscale patterning of a large variety of materials, from metals to semiconductors; from self-assembled monolayers to biomolecules. Oxidation SPL has also been applied to develop sophisticated electronic and nanomechanical devices such as quantum dots, quantum point contacts, nanowire transistors or mechanical resonators. Here, we review the principles, instrumentation aspects and some device applications of o-SPL. Our focus is to provide a balanced view of the method that introduces the key steps in its evolution, provides some detailed explanations on its fundamentals and presents current trends and applications. To illustrate the capabilities and potential of o-SPL as an alternative lithography we have favored the most recent and updated contributions in nanopatterning and device fabrication.

Tip-Based Nanofabrication for Scalable Manufacturing

Tip-based nanofabrication (TBN) is a family of emerging nanofabrication techniques that use a nanometer scale tip to fabricate nanostructures. In this review, we first introduce the history of the TBN and the technology development. We then briefly review various TBN techniques that use different physical or chemical mechanisms to fabricate features and discuss some of the state-of-the-art techniques. Subsequently, we focus on those TBN methods that have demonstrated potential to scale up the manufacturing throughput. Finally, we discuss several research directions that are essential for making TBN a scalable nano-manufacturing technology.

Site-Specific Chemical Surface Functionalization and Electronic Patterning of Graphene by Electrooxidative Lithography

The combination of different properties being manipulated on nanomaterials is one of the challenges in nanotechnology research. In particular, the possibility to tailor the electronic and chemical properties offers promising possibilities for the design of functional nanostructures. Herein, we report an approach that permits control of these properties on the basis of electrooxidative lithography to structure reduced graphene oxide functionalized with a self-assembled monolayer of n-octadecyltrichlorosilane. The electrochemical oxidation process first induces the formation of polar acid groups on the monolayer, which can be used to covalently bind nanoparticles and molecules and, secondly, also allows the reoxidation of the underlying reduced graphene oxide. As such, the chemical signature as well as the electronic properties of the substrate can be tailored on the micro- and nanometer scale. Details on the oxidation of the monolayer as well as thorough characterization of the electronic properties will be presented. Finally, the approach is used to demonstrate the fabrication of a sensitive glucose sensor device.

Investigation of the Transition from Local Anodic Oxidation to Electrical Breakdown During Nanoscale Atomic Force Microscopy Electric Lithography of Highly Oriented Pyrolytic Graphite

As one of the tip-based top-down nanoscale machining methods, atomic force microscopy (AFM) electric lithography is capable of directly generating flexible nanostructures on conductive or semi-conductive sample surfaces. In this work, distinct fabrication mechanisms and mechanism transition from local anodic oxidation (LAO) to electrical breakdown (BD) in the AFM nanoscale electric lithography of the highly oriented pyrolytic graphite sample surface was studied. We provide direct evidence of the transition process mechanism through the detected current-voltage (I-V) curve. Characteristics of the fabrication results under the LAO, transition, and BD regions involving the oxide growth rate or material removal rate and AFM probe wear are analyzed in detail. These factors are of great significance for improving the machining controllability and expanding its potential applications.

3D flexible water channel: stretchability of nanoscale water bridge

Artificial water channels can contribute to a better understanding of natural water channels and offer a highly selective, advanced conductance system. Most studies use nanotubes, however it is difficult to fabricate a flexible structure, and the nanosized diameter brings nanoconfinement effects, and nanotube toxicity arouses biosafety concerns. In this paper, we use an electric field to restrain the water molecules to form a nanoscale water bridge as an artificial water channel to connect a separated solid plate by molecular dynamics simulations. We observe strong 3D flexible stretchability in the water bridge, maintaining a variable length and an arbitrary angle for a considerably long time. The stretching of the water bridge enables it to be polarized at an arbitrary angle and the stretchability is linearly dependent upon the polarization strength. More interestingly, we show the possibility of establishing complex water networks, e.g., triangle, rectangle, hexagon, and tetrahedron-tetrahedron water networks. Our results may help realize structurally flexible and environmentally friendly water channels for lab-on-a-chip applications in nanofluidics.

The role of ambient ice-like water adlayers formed at the interfaces of graphene on hydrophobic and hydrophilic substrates probed using scanning probe microscopy

In this work, we report the role of ice-like water adlayers (IWLs) formed under ambient conditions in between mechanically exfoliated as-prepared and patterned few layer graphene (FLG) and multi-layer graphene (MLG) on hydrophobic Si and hydrophilic SiO2/Si substrates. The growth of the IWL is probed by measuring the height changes in graphene using intermittent contact atomic force microscopy (IC-AFM) and their electrostatic effect is studied using electrostatic force microscopy (EFM) over time. It is found that more IWLs are formed within a shorter period of time, when both as-prepared graphene and underlying substrates are either hydrophobic or hydrophilic in nature. In contrast, AFM voltage nanolithographically patterned trenches on FLG and MLG on the Si substrate show quick formation of IWLs. The effect of IWL formed, on the dimensions of trenches, is correlated with the variation of the measured EFM phase shift over time. This study demonstrates the dependence of the formation of IWLs under ambient conditions on the affinity towards water, at the interface of graphene on hydrophobic and hydrophilic substrates, which has important implications for the performance of graphene-based nanoelectronic devices.

Nanotechnologies in protein microarrays

Protein microarray technology became an important research tool for study and detection of proteins, protein-protein interactions and a number of other applications. The utilization of nanoparticle-based materials and nanotechnology-based techniques for immobilization allows us not only to extend the surface for biomolecule immobilization resulting in enhanced substrate binding properties, decreased background signals and enhanced reporter systems for more sensitive assays. Generally in contemporarily developed microarray systems, multiple nanotechnology-based techniques are combined. In this review, applications of nanoparticles and nanotechnologies in creating protein microarrays, proteins immobilization and detection are summarized. We anticipate that advanced nanotechnologies can be exploited to expand promising fields of proteins identification, monitoring of protein-protein or drug-protein interactions, or proteins structures.

Advanced scanning probe lithography

The nanoscale control afforded by scanning probe microscopes has prompted the development of a wide variety of scanning-probe-based patterning methods. Some of these methods have demonstrated a high degree of robustness and patterning capabilities that are unmatched by other lithographic techniques. However, the limited throughput of scanning probe lithography has prevented its exploitation in technological applications. Here, we review the fundamentals of scanning probe lithography and its use in materials science and nanotechnology. We focus on robust methods, such as those based on thermal effects, chemical reactions and voltage-induced processes, that demonstrate a potential for applications.