Minhua Zhao


National Research Council, NIST/NIH Associateship Program


Scanning surface potential microscopy is a powerful tool for measuring surface potential at the nanoscale. However, it is still a qualitative method due to principle of the measurement. All three parts of an AFM probe (tip apex, tip cone and cantilever) will interact with the sample owing to the long-range interaction of electrostatic force. Hence, the measured potential is not equal to true local surface potential, limiting both the spatial and potential resolution of the measurement. Here we demonstrate a more quantitative potential measurement by employing two types of high aspect ratio probes, which greatly reduce the non-local electrostatic interaction between the tip and sample. One is carbon nanotube AFM probes prepared by dielectrophoresis. The unique feature is both high aspect ratio (>30:1 compared to ~3:1 for standard AFM probe) and ultra sharp at the end (radius <10 nm). Compared to standard AFM probes, enhancement of potential contrast by several factors is reported. Particularly, surface potential measurement on Bacteriorhodopsin proteins reveals better than 2nm in spatial resolution, the best reported results by this technique under ambient condition. The other type of probes used is Ag2Ga nanowire probes, prepared by growing Ag2Ga nanowire on the AFM probe. The length of the nanowire can be adjusted from several to tens of micrometers with the diameter from tens to hundreds of nanometers. Surprisingly enhanced surface potential measurement on single wall carbon nanotubes and high aspect ratio micro-patterned features is demonstrated by nanowire probes compared to that by standard AFM probes.  Quantitative surface potential measurement extends the capability of AFM beyond topographic imaging, with promising applications in semiconductor industry and biotechnology, such as doping analysis and label-free protein detection.




Author Information:


Name:                           Minhua Zhao

Mentor’s name:            Tinh Nguyen

Division:                       Materials and Construction Research

Laboratory:                   BFRL

Room/Building:            B358/226

Mail Stop:                     8615

Telephone #:                 301-975-8923

Fax #:                            301-990-6891

Email:                            minhua.zhao@nist.gov

Sigma Xi:                      Yes

Category:                       Materials