DTIC ADA299096: Tunneling Spectroscopy of Ultrasmall Clusters and Grains.

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1. AGENCY USE ONLY {Leave Blank) 2. SEPORT DATE

August 1995

4. TITLE AND SUBTITLE

3. REPORT TYPE AND DATES COvsncw

Final Technical Report, 7/-1/92-6/30 ^95

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Tunneling Spectroscopy of Ultrasmail Clusters and Grain:

S.ALrTHORiS)

K. Likharev

7 performing organization NAME(S) and ADDR£SS(ES)

state University of New York at Stony Brook
Stony Brook, NY 11794-3800

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8. PERFORMING ORGANIZATION
REPORT NUMBER

9. SPONSORING/MONITOflING AGENCY NAME(S) AND ADDRESS(ES)

AFOSR'A’^ jj- ”7VT

Dr. Harold Weinstock I • 8

110 Duncan Ave.

Bolling AFB, DC 20332-0001

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Approved for public release ;

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13. ABSTRACT {M^ximum 200 wonis)

The Final Technical Report describes
project supported by an AFOSR AASERT
#AFOSR-91-0445). The report consists

1. Summary

2. Main Results

3. Conclusion and Prospects.
The main research achievement during
original instrument for scanning tun
electronic remote control of a I I its
of the scanning tip), for a broad ra

10. SPONSORINGrtWONITORINQ
AGENCY REPORt NUMBER

il2b. DISTRIBUTION CODE

19951002 018 _

resuiTs obtained during the research
Grant #F49620-92-J-0358 (parent grant
of 3 sections:

this work has been the development of an
neling microscopy and spectroscopy, with
functions (including the coarse approach
nge of research applications.

DTIC QUALITY INSPECTED 8

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14. SUBJECT TERMS

Scanning tunneling microscopy, tunneling spectroscopy, 16 .PRICECODE

s i ng I e-e 1 ectr onics, nano-obJects, cluster s, thin films mntAtifiM JS* 7ra2

NSN 7540-01-280-5500

Standard Form 296 (Rav. 2-89

I sUI

TUNNELING SPECTROSCOPY
OF ULTRASMALL CLUSTERS AND GRAINS

AFOSR Grant # F49620-92-J-0358

(FY91 AASERT)

Parent Grant # AFOSR-91-0445

Final Technical Report

Principal Investigator: Prof. Konstantin K. Likharev

Address: Department of Physics

State University of New York
Stony Brook, NY 11794-3800

Phone: 516-632-8159
Fax: 516-632-8774

e-mail: KLIKHAREV@CCMAILSUNYSB.EDU

Project Period: July 1, 1992 - June 30, 1995

August 1995

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1. Summary

Single-electronics is a new and exciting field of physical and applied
electronics, which has emerged during the last decade. The physics of this field is based
on the effects of correlated single-electron tunneling. Their essence is that the transfer of
single electrons in systems of conducting (metallic, semiconductor, or molecular)
"electrodes", connected by tunnel barriers of very small area, may be strongly correlated
either in time, or in space, or both.

Since 1987, reliable evidence of correlated tunneling has been obtained in
numerous experiments with normal-metal, superconductor, and semiconductor junctions
and systems. Preliminary studies of possible applications have shown that single¬
electronics may yield a completely new generation of both digital and analog devices
with unparalleled performance, most notably extremely dense digital circuits with up
to loll active devices (logic gates and/or memory cells) per square centimeter.
However, in order to bring single-electronics to practice, numerous problems must be
solved. In 1991, three research groups at SUNY - Stony Brook, headed by Professors
Dmitri Averin, Konstantin Likharev and James Lukens, working in collaboration, began
an AFOSR-supported project in the field of single-electronics. As a result of this effort, a
solid technological, experimental and theoretical base for single-electronics was
established at Stony Brook, and several important results have been obtained.

The main objectives of the present supplementary project have been:

- to extend the work to scanning tunneling microscopy/spectroscopy (STM/S) of
various nanostructures (ultimately at low temperatures), and

- to increase the number of graduate students involved in research in the field of
single-electronics.

During the first two years of the project, the research was carried out in
collaboration with the group of Dr. Myron Strongin at the Brookhaven National
Laboratory. The collaboration was very pleasant and fruitful, and provided a substantial
leverage for our AASERT award.

Our main research achievement has been the development of an original STM/S
instrument with electronic remote control of all its functions (including the coarse
approach), with a broad range of possible applications. This instrument has been used to
carry out STM studies of a broad variety of nanostructures, including Moire patterns on
graphite surfaces, monolayers of stearic acid on graphite, atomically-smooth gold thin
films on mica, monolayers of C 50 buckybaUs on atomically-smooth gold surfaces (in
particular, modified by extremely thin gold overlayers), and metal-doped diamond-like
carbon.

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A more detailed description of the main results of this work is given below. ■

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2. Main Results

A. Simulation of Interaction of Single-Electron Solitons

For adequate interpretation of anticipated experimental results on single-electron
charging of ID arrays of nano-objects, we have carried out analytical and numerical
calculations of interaction of single-electron solitons in such arrays, using simple
geometrical models. In contrast to previous calculations which had used oversimplified
assumptions, we have found a natural crossover between the exponential decrease of the
electrostatic field near the center of the soliton and the virtually unshielded (1/r) decrease
of the scalar potential at large distances. A simple expression for the stray capacitance
per period has been derived from these numerical results [1].

B. STM/S Instrument with New Piezoelectric Engine

For our single-electron nanospectroscopy work we would like to have an STM/S
instrument which could operate at helium and millikelvin temperatures and high vacuum
without any mechanical links to the ambient environment. (Commercially available low-
temperature STMs are not applicable in our experimental conditions). This is why we
have designed and implemented a preliminary version of such an STM/S using a new
type of piezoelectric engine for the tip position adjustment [2].

The engine consists of a 60° quartz prism (carrying the piezo-tube STM scanner)
which can move along a 60° cut in a Macor body of the sample holder. A spring presses
the prism against 4 shear-motion piezo plates ("legs") which are glued to the body.
During the first stage of each crude approach step, the voltage on all the legs is changed
simultaneously. The upper surfaces of the prisms shift along the cut, and carry the prism
with them. Then the voltage is removed from the legs one-by-one; during this stage the
prism stays in place, because at each instant its friction against 3 static legs overweighs
the friction against 1 moving leg.

Our engine, and the STM/S as a whole, operates reliably at room temperatures
(both in air and high vacuum) and has been applied to studies of several nanosystems
which seemed promising for the observation of correlated single-electron tunneling
effects.

C. STM/S Studies of Nanostructures

C.l. Moire patterns on HOPG

Highly oriented pyrolythic graphite (HOPG) is a commonly used substrate for
STM/S applications. Using our STM/S we could observe atomic patters on the HOPG
surfaces, including the Moird patterns associated with variations of local stacking [2],
without any vibration isolation. We have discovered an extended electronic effect from a
single atomic vacancy, and measured the decay factor along the c-axis and various bias
voltage dependent topographies for the Moird patterns [3]. These results may be very
useful for further studies of ultrasmall particles and clusters on HOPG substrates.

C.2. Macromolecules on HOPG

To fix ultrasmall conducting particles on a graphite surface, Langmuir-Blodgett
films of stearic acid mixed with l, 7 -(CH 3 ) 2 -l, 2 -C 2 BioHgTl(C)CC)CF 3)2 molecules
were prepared using distilled water, and transferred onto freshly cleaved HOPG substrate
using the Langmuir-SchSfer method. Our STM study has shown that the surfaces of the
graphite substrates had been modified. We have determined that the modification was
caused by the solvent intercalation. While being detrimental for our particular attempt to
fix organic clusters on the surface, this effect may be useful in general to characterize
surface damage caused by different sample preparation techniques.

C.3. Au (111) and

Gold films were deposited at 440°C on pre-baked mica substrates. Using our
STM we could get atomic resolution on these films in air. The STM studies have shown
that atomically flat regions on the films were ~0.1 |im wide. By evaporation of
buckyballs (C 50 ) on the surface we could obtain and image well-ordered single-
molecular layers with hexagonal structure [4]. To our surprise, the morphology of these
structures changes dramatically after adding one more Au overlayer. While the first C 50
monolayer remained flat, the extra buckyballs formed clusters with an average size of 8
nm.

C. 4, Some other applications

Our STM/S instrument has been used for some purposes beyond the scope of this
project (the corresponding research costs were covered from funds different from the
AASERT award). For example, the BNL group have used this instrument to study metal-
containing diamond-like nanocomposites (a-C:H)/(a-Si:0)/Me films [ 4 ]. The electrical
and structural properties of these films could be controlled by several means, e.g., metal
doping, therm^ annealing and probably high field (or heating) in the vicinity of an STM
tip. Using STM, the annealing- induced sp^-to-sp^ transformation (which may provide
an interesting way to engineer nanostructured samples) has been observed [ 4 ].

3. Conclusion and Prospects

Unfortunately, we have not been able to find sufficiently stable objects which
could exhibit correlated single-electron tunneling effects at room temperature. On the
other hand, low-temperature (~5 K) operation of our instrument was not stable enough
for its systematic use. Nevertheless, the instrument has turned out to be extremely useful
for several room-temperature studies of issues closely related to the future development
of nanoelectronics. Our current plans are to use it mostly for in-situ studies of the
thermal growth of very thin (~ 2 -nm) aluminum oxide layers used as tunnel barriers in
niobium-trilayer tunnel junctions for our single-electronics research within our parent
AFOSR project, as well as for superconductor electronics work funded by DoD's URI
through another AFOSR grant.

4.-References

1. K.K. Likharev K. Matsuoka, "Electron-Electron Interaction in Linear Arrays
of Small Tunnel Junctions", submitted to Appl. Phys. Lett. (July 1995).

2. Z.Y. Rong and P. Kuiper, "Electronic Effects in Scanning Tunneling Microscopy:
Moird Pattern on a Graphite Surface", Phys. Rev. B 48,17427 (1993).

3. Z.Y. Rong, "Extended Modifications of Electronic Structures Caused by Defects:
Scanning Tunneling Microscopy of Graphite", Phys. Rev. B 50, 1839 (1994).

4. Z.Y. Rong and L. Rokhinson, "STM Study of Gold-overlayer Formation on CgQ
Monolayers", Phys. Rev. B 49,7749 (1994).

5. Z.Y. Rong, M. Abraizov, B. Dorfman, M. Strongin, X.-Q. Yang, D. Yan, and F.
H. Poliak, "Scanning Tunneling Microscopy of Diamond-like Nanocomposite
Films", Appl. Phys. Lett. 65, 1379 (1994).