Scanning tunneling microscopy of 1-D and 2-D nanomaterials

Scanning tunneling microscopy (STM) is a technique that enables atomic resolution imaging of conducting and semiconducting surfaces. Together with scanning tunneling spectroscopy (STS), STM is used to image and measure the electronic properties of surfaces, molecules, and nanomaterials. The Lyding group investigates a wide variety of 1-D and 2-D materials including graphene, graphene nanoribbons, and carbon nanotubes. STS is used to obtain detailed electronic information which is essential when evaluating candidates for high performance electronic devices. the role is surface and any adsorbed molecules are sensed by a tunneling current -- making investigation of surface physics on atomic scale possible. Please click on the images to learn more:

Atomically Precise Graphene Nanoribbons

With increasingly demanding miniaturization and performance requirements in the electronics industry, atomically precise graphene materials may play a significant role in high-performance computing.  A previous scanning tunneling spectroscopy study published by the Lyding group in Nature Materials experimentally determined that structural details at the atomic scale have a large effect on the electronic properties of graphene nanostructures. As a result, atomic precision is required to obtain well-controlled electronic properties. Through the bottom-up approach nanoribbons can be made with atomic precision, enabling control over their electronic structure. 

STM Topograph of an Atomically precise chevron graphene nanoribbon on H:Si(100)

Graphene nanoribbons are synthesized either on metal surfaces using molecular precursors, or synthesized in solution as demonstrated by the Sinitskii group, our collaborators at the University of Nebraska-Lincoln. Previously, several groups have shown that nanoribbons can be transferred from the metal substrates onto other substrates using wet chemical methods, but high-quality images and electronic measurements could not be obtained due to problems associated with solvent residue. Our work, recently published in Nano Letters (DOI: 10.1021/acs.nanolett.6b03709), shows that solution-synthesized graphene nanoribbons can be cleanly placed onto arbitrary surfaces. 

Development of high strength CNT fibers and next generation CNT transistors

Carbon nanotubes (CNT) are explored extensively because of their exceptional electrical, thermal, chemical and mechanical properties. The small dimensions annd mechanical flexibility of CNTs make them potentially useful as the active channel materials in thin-film transistors (TFTs). Chemical vapor deposition of CNTs can result in TFTs with mobilities of over 1000 cm^2 V-1 s-1 and on/off current ratios as high as 10^5. 

While CNT networks are relatively easy to fabricate and exhibit promising performance, they have drawbacks. Carrier mobility, conductivity and power dissipation are limited by resistance at CNT junctions. Studies have shown that the electrical and thermal resistances at the CNT junctions are an order of magnitude higher than those of individual CNTs. Passing current through these junctions causes localized heating which degrades device performance and reliability.

In this project, we focus on solution based techniques to deposit nanoparticles selectively at these CNT junctions. Our process involves spin-coating a chemical precursor from a volatile solvent to form a thin film of the precursor on top of the CNT network. Passing current through these networks causes localized heating at the CNT junctions which induces thermal decomposition of the precursor. Our previous results have shown that depositing metallic (Pd) particles at these junctions improves the Ion/Ioff ratio and the mobility of CNT networks by a factor of 6. We are currently working to perfect and improve this technique by investigating alternative precursor materials.

Do et al. Copyright © 2013 American Chemical Society

Do et al. Copyright © 2013 American Chemical Society

Atomically precise doping of Si

This project focuses on using the STM for atomically precise placement of dopant atoms on the Si surface. We utilize hydrogen lithography, in which the STM tip selectively removes hydrogen atoms that were bonded to the surface Si atoms. This creates reactive dangling bonds, on which gaseous precursors may interact to introduce dopant atoms.

Glassy dynamics

Glasses have been used for thousands of years, covering a wide range of applications from daily appliances to optical, electronic and medical devices. However, the microscopic understanding of glasses and the glass transition remains a highly active and controversial area. In addition to the bulk glass, studying surface glass dynamics likely contributes equally towards understanding the connection between theories of glass and experiments to test these theories.

We study surface glassy dynamics by making STM movies of various amorphous surfaces at room temperature, including Ce- and La-based metallic glasses, amorphous silicon carbide and amorphous hafnium diboride. Surface glassy dynamics under external excitations including temperature and light irradiation is also investigated.  

NGUYEN ET AL. JCP 2014, Copyright 2014 © AIP PUBLISHING LLC

NGUYEN ET AL. JCP 2014, Copyright 2014 © AIP PUBLISHING LLC

Single molecule absorption

We use optically assisted STM to study absorption and intermolecular energy transfer between single quantum dots (QDs) and carbon nanotubes (CNTs). Single PbS, CdSe, CdSe/ZnS, CdSSe/ZnS QDs and CNTs are deposited onto gold, crystalline silicon carbide (c-SiC) and a-SiC surfaces by matrix-assisted dry contact transfer. Adsorbed molecules are excited with modulated laser light and the modulated tunneling current proportional to the optical absorption signal is detected by STM with a lock-in amplifier.

Nienhaus et al. JACS 2015,  Copyright © 2015 American Chemical Society

Nienhaus et al. JACS 2015,  Copyright © 2015 American Chemical Society

Plasmonics in metals and semiconductors

We are exploring the generation, transmission and dispersion of plasmons in nanoscale metals and semiconductors. For this purpose, we are developing low-cost scalable fabrication techniques towards applications in light harvesting, field-effect transistor performance, tunable sensing, photodetectors and spectroscopy. (Image scale: 5 microns x 2 microns)

Gold bowties on silicon, copyright © 2016 Lyding GROUp

Gold bowties on silicon, copyright © 2016 Lyding GROUp

Low temperature ultra-high vacuum scanning tunneling microscope

Compared to room temperature STM, scanning at cryogenic temperatures can reduce the thermal drift as well as freeze those absorbates. Therefore resolution can be improved and makes manipulation of atoms and molecules by the tip much easier. We are currently building a low temperature ultra-high vaccum scanning tunneling microscope (LT-UHV-STM) which is capable of scanning at a temperature of below 10 Kelvin. A novel design of the cooling mechanism was implemented, which is to use a closed-cycle helium-based refrigerator to cool the system.