Tuanwei Shi

Transversally and axially tunable carbon nanotube resonators in situ fabricated and studied inside a scanning electron microscope.

We report a new design of carbon nanotube (CNT) resonator, whose resonance frequency can be tuned not only transversally by a gate voltage, but also by the axial strain applied through directly pulling the CNT. The resonators are fabricated from individual suspended single-walled CNT (SWCNT) in situ inside a scanning electron microscope. The resonance frequency of a SWCNT resonator can be tuned by more than 20 times with an increase of quality factor when the axial strain of the SWCNT is only increased from nearly zero to 2% at room temperature. The transversal gate-tuning ability is found to be weaker than the axial-tuning ability and decrease with increasing the axial strain. The gate voltage can hardly tune the resonance frequency when the initial axial strain is larger than 0.35% and the CNT acts like a tied string. The relationship among resonance frequency, gate voltage, and initial axial strain of the CNT obtained presently will allow for the designs of CNT resonators with high frequency and large tuning range. The present resonator also shows ultrahigh sensitivity in displacement and force detection, with a resolution being better than 2.4 pm and 0.55 pN, respectively.

Electrical characteristics of field-effect transistors based on indium arsenide nanowire thinner than 10 nm

To suppress short channel effects, lower off-state leakage current and enhance gate coupling efficiency, InAs nanowires (NWs) with diameter smaller than 10 nm could be needed in field-effect transistors (FETs) as the channel length scales down to tens of nanometers to improve the performance and increase the integration. Here, we fabricate and study FETs based on ultrathin wurtzite-structured InAs NWs, with the smallest NW diameter being 7.2 nm. The FETs based on ultrathin NWs exhibit high Ion/Ioff ratios of up to 2 × 108, small subthreshold swings of down to 120 mV/decade, and operate in enhancement-mode. The performance of the devices changes as a function of the diameter of the InAs NWs. The advantages and challenges of the FETs based on ultrathin NWs are discussed.

Towards on-chip time-resolved thermal mapping with micro-/nanosensor arrays

In recent years, thin-film thermocouple (TFTC) array emerged as a versatile candidate in micro-/nanoscale local temperature sensing for its high resolution, passive working mode, and easy fabrication. However, some key issues need to be taken into consideration before real instrumentation and industrial applications of TFTC array. In this work, we will demonstrate that TFTC array can be highly scalable from micrometers to nanometers and that there are potential applications of TFTC array in integrated circuits, including time-resolvable two-dimensional thermal mapping and tracing the heat source of a device. Some potential problems and relevant solutions from a view of industrial applications will be discussed in terms of material selection, multiplexer reading, pattern designing, and cold-junction compensation. We show that the TFTC array is a powerful tool for research fields such as chip thermal management, lab-on-a-chip, and other novel electrical, optical, or thermal devices.

Contact properties of field-effect transistors based on indium arsenide nanowires thinner than 16 nm.

With the scaling down of field effect transistors (FETs) to improve performance, the contact between the electrodes and the channel becomes more and more important. Contact properties of FETs based on ultrathin InAs NWs (with the diameter ranging from sub-7 nm to 16 nm) are investigated here. Chromium (Cr) and nickel (Ni) are proven to form ohmic contact with the ultrathin InAs NWs, in contrast to a recent report (Razavieh A et al ACS Nano 8 6281). Furthermore, the contact resistance is found to depend on the NW diameter and the contact metals, which between Cr and InAs NWs increases more rapidly than that between Ni and InAs NWs when the NW diameter decreases. The origins of the contact resistance difference for the two kinds of metals are studied and NixInAs is believed to play an important role. Based on our results, it is advantageous to use Ni as contact metal for ultrathin NWs. We also observe that the FETs are still working in the diffusive regime even when the channel length is scaled down to 50 nm.

A platform for in-situ multi-probe electronic measurements and modification of nanodevices inside a transmission electron microscope

We developed a new platform that enables in-situ four-probe electronic measurements, in-situ three-probe field-effect measurements, nanomanipulation, and in-situ modification of nanodevices inside a transmission electron microscope (TEM). The platform includes a specially designed chip-holder and a silicon (Si) chip with suspended metal electrodes. The chip-holder can hold one Si chip with a size up to 3 mm × 3 mm and provides four electrical connections that can be connected to the micrometer-sized electrodes on the Si chip by wire-bonding. The other side of the electrical connections on the chip-holder is connected to the electronic instruments outside the TEM through a commercial Nanofactory SPM-TEM holder. The Si chip with suspended metal electrodes on one of its edges was fabricated by lithography and wet etching. Carbon nanotubes (CNTs), InAs nanowires, and tungsten disulfide nanowires were placed to stride over and connect to the suspended electrodes on the Si chip by nanomanipulations inside a scanning electron microscope (SEM). By using the platform, I-V curves of an individual single-walled CNT connecting to four electrodes were in-situ measured between any two of the four suspended electrodes, and a high-resolution TEM image of the same CNT was obtained. Furthermore, four-terminal I-V measurement on an InAs nanowire was achieved on this platform, and with a movable probe used as a gate electrode, field-effect measurement on the same InAs nanowire device was accomplished in SEM. In addition, by using the movable probe on the SPM-TEM holder, we could further in-situ modify nanomaterial and nanodevices. The present work demonstrates a method that allows a direct correlation between the atomic-level structure and the electronic property of nanomaterials or nanodevices whose structure can be further modified in-situ.

In situ multiproperty measurements of individual nanomaterials in SEM and correlation with their atomic structures.

The relationship between property and structure is one of the most important fundamental questions in the field of nanomaterials and nanodevices. Understanding the multiproperties of a given nano-object also aids in the development of novel nanomaterials and nanodevices. In this paper, we develop for the first time a comprehensive platform for in situ multiproperty measurements of individual nanomaterials using a scanning electron microscope (SEM). Mechanical, electrical, electromechanical, optical, and photoelectronic properties of individual nanomaterials, with lengths that range from less than 200 nm to 20 μm, can be measured in situ with an SEM on the platform under precisely controlled single-axial strain and environment. An individual single-walled carbon nanotube (SWCNT) was measured on the platform. Three-terminal electronic measurements in a field effect transistor structure showed that the SWCNT was semiconducting and agreed with the structure characterization by transmission electron microscopy after the in situ measurements. Importantly, we observed a bandgap increase of this SWCNT with increasing axial strain, and for the first time, the experimental results quantitatively agree with theoretical predictions calculated using the chirality of the SWCNT. The vibration performance of the SWCNT, a double-walled CNT, and a triple-walled CNT were also studied as a function of axial strain, and were proved to be in good agreement with classical beam theory, although the CNTs only have one, two, or three atomic layers, respectively. Our platform has wide applications in correlating multiproperties of the same individual nanostructures with their atomic structures.

Nanoscale opening fabrication on Si (111) surface from SiO2 barrier for vertical growth of III-V nanowire arrays

We reported here a selectively additive process to fabricate nanoscale openings of an Si (111) surface from an SiO2 barrier layer. Such nanoscale openings are made for the growth of vertical III-V nanowires. The Si (111) surface protected by a patterned SiNx layer was thermally oxidized, which resulted in a selectively added SiO2 barrier layer. After removing the SiNx, nanoscale openings of the Si (111) surface were exposed for the nanowire growth. Arrays with patterned nanoholes of varied diameters from 60 nm to 334 nm have been used for position-controlled catalyst-free growth of vertical InAs nanowire arrays by metal-organic chemical vapor deposition. Correlations between the nanohole diameter and the diameter, length and growth yield of as-fabricated nanowire arrays have been investigated, showing a repeatable stability. This technique offers an alternative approach for the fabrication of novel III-V nanowire devices using vertical array configuration. A lateral thermal oxidation effect led to a smaller size of the Si opening than that of the SiNx protection nanoislands; therefore, the technique also offers a controllable way to produce nanoholes with an ultra-small diameter.