Tip position modulation and lock-in detection in scanning electrochemical microscopy

• US 5,382,336 A
• Filed: 04/16/1992
• Issued: 01/17/1995
• Est. Priority Date: 04/16/1992
• Status: Expired due to Fees

• Current Assignee: Board of Regents of the University of Texas System (University of Texas System)
• Original Assignee: Board of Regents of the University of Texas System (University of Texas System)
• Inventors: Wipf, David O., Bard, Allen J.
• Primary Examiner(s): Niebling, John
• Assistant Examiner(s): Bell, Bruce F.
• Application Number:
  US07/869,301
• Time in Patent Office: 1,006 Days
• Field of Search: 204/153.1, 204/400
• US Class Current: 205/790.5
• CPC Class Codes: B82Y 35/00 Methods or apparatus for measurement or analysis of nanostructures
• CPC Class Codes: G01Q 60/60 SECM [Scanning Electro-Chemical Microscopy] or apparatus therefor, e.g. SECM probes
• CPC Class Codes: Y10S 977/852 for detection of specific nanostructure sample or nanostructure-related property

Abstract

Small amplitude tip-position modulation (TPM) in combination with lock-in detection of the modulated current signal is applied to a scanning electrochemical microscope (SECM) to improve signal-to-noise ratio and to enhance image resolution. Phase shift information from the alternating current TPM signal and the bipolar response of TPM over insulators and conductors make TPM-SECM superior to conventional SECM imaging methods as well as useful for measuring substrate conductivity.

Claims

1. A method for characterizing a substrate surface, the method comprising

establishing an electrical potential between a reference electrode and an ultramicroelectrode, the ultramicroelectrode being separated by a distance from a substrate surface and in communication with the surface through an electrolyte solution comprising a mediator;

and

measuring electrical current changes in the ultramicroelectrode as a function of continuous or discrete variations in the distance separating the ultramicroelectrode from the surface as the ultramicroelectrode is scanned over the surface;

wherein the current is faradaic, decreasing with increasing distance when the surface is a conductor and increasing with increasing distance when the surface is a insulator.

2. The method of claim 1 wherein the substrate surface comprises a conducting area and further comprising the step of

establishing an electrical substrate potential between a conducting substrate and the reference electrode,

wherein the substrate potential is sufficiently positive with respect to the mediator redox potential so that a substrate reaction at the tip is diffusion limited.

3. The method of claim 1 wherein the distance separating the ultramicroelectrode from the surface is varied sinusoidally at frequency f_m to produce simple harmonic motion of the ultramicroelectrode with respect to the surface and an alternating current of frequency f_m in the ultramicroelectrode.

4. The method of claim 3 wherein the simple harmonic motion has a frequency between about 20 Hz and about 20,000 Hz.

5. The method of claim 1 wherein the distance separating the ultramicroelectrode from the surface is varied discretely.

6. The method of claim 1 wherein the ultramicroelectrode has a conductor radius a'"'"' of between about 0.005 and 12.5 μ

m.

7. The method of claim 6 wherein the ultramicroelectrode moves in simple harmonic motion through distance δ

perpendicular to the surface at a distance of about d'"'"' from the surface;

wherein d'"'"'>

about 2.5δ

; and

wherein δ

/a'"'"' is between about 0.01 and 1.0.

8. The method of claim 1 wherein the mediator is Ru(NH₃)₆ Cl₃, Fe(CN)₆^3-, Fe₂ (SO₄)₃, or methyl viologen dication.

9. The method of claim 1 wherein the mediator is at a concentration from about 0.1 mM to about 2.0M.