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1000 P. Sutter

are immobile on the adsorbate-free surface. Vacancy diffusion is greatly

enhanced by adsorbed O

. At temperatures sufﬁ ciently low that the

diffusion of adsorbed O

molecules can be captured by STM, the sub-

traction of consecutive frames in time-lapse STM shows that single

oxygen vacancies diffuse along [11

0] from one bridging O row to the

next, always in the presence of neighboring O

molecules.

The role of O

molecules in the vacancy diffusion process is estab-

lished from detailed investigation of single vacancy hops, again based

on time-lapse STM movies at low temperature. As an O

molecule dif-

fusing along a Ti row approaches an oxygen vacancy, it dissociates and

contributes one oxygen atom toward healing the vacancy, thus creating

a metastable intermediate consisting of a single O atom. The O adatom

is highly reactive, as corroborated in separate experiments involving

dosing of atomic oxygen. It rapidly recombines with a bridging O atom

and emerges as an O

molecule. If in this process the bridging O atom

is removed from one of the adjacent rows, the net result is a diffusion

jump of an oxygen vacancy by one bridging oxygen row. Given this

-mediated mechanism of oxygen vacancy diffusion, the rate of dif-

fusion events is expected to scale linearly with O

coverage. STM

movies obtained at different O

exposure show that this is indeed the

case.

While early imaging of dynamic surface processes was performed

almost invariably in UHV, several applications require STM imaging

in what is seen as more “realistic” environments for those applications.

A prominent example is heterogeneous catalysis. It has been recog-

nized that actual reactions under technologically relevant conditions,

often involving elevated temperatures and pressures at or above atmo-

spheric pressure, can involve surface structures and compositions, and

entire reaction mechanisms that differ substantially, even qualitatively,

from those of “simulated” reactions running in UHV, a situation com-

monly termed the “pressure gap” problem of heterogeneous catalysis.

To address the need for imaging with high spatial and temporal resolu-

tion at elevated pressure, a family of dedicated STM instruments was

developed (Rasmussen et al., 1998; Jensen et al., 1999; Lægsgaard et al.,

2001; Rößler et al., 2005). These instruments allow sample preparation

and surface analysis in UHV, followed by exposure to reactants at high

pressure and simultaneous STM imaging. A particularly elegant imple-

mentation of this concept is the “reactor STM,” allowing dynamic STM

imaging of surfaces exposed to reactants in a compact catalytic ﬂ ow

reactor in combination with the simultaneous analysis of the reaction

products by mass spectrometry.

Figure 15–23 shows an example of a complex dataset obtained during

high-pressure CO oxidation on Pt(110) (Hendriksen and Frenken,

2002). The upper panel traces mass spectrometer signals for O

, CO,

and CO

, showing initial exposure to CO, followed by the introduction

of molecular oxygen into the reactor. The panel below shows represen-

tative STM images obtained at speciﬁ c stages of the reaction, during

which the sample is kept at a constant temperature of 425 K. Images A,

B, E, F, and H show ﬂ at terraces separated by steps, representing the

metallic, CO-covered Pt(110) surface. Image C shows the change in

Chapter 15 Scanning Tunneling Microscopy in Surface Science 1001

surface morphology, a pronounced surface roughening, during a step

in activity giving rise to a sudden increase in CO

evolution, demon-

strating a direct link between surface roughness and activity for this

surface involving a mechanism that is not observed at low pressure.

Although capable of mapping dynamic surface phenomena, STM

movies have obvious limitations in imaging fast dynamic processes,

such as surface diffusion at room temperature or above. A possible

solution is the development of novel approaches and instruments for

very high-speed STM imaging. As an alternative, frame-by-frame

imaging can be abandoned altogether if only a map of the trajectory

of the diffusing species is desired. The recognition of this fact led to

the development of atom tracking STM (Swartzentruber, 1996). In atom

tracking, the STM tip is locked onto a diffusing surface species using

a 2D lateral feedback mechanism. Once locked, the feedback maintains

Figure 15–23. Simultaneous mass spectrometry and STM imaging in a catalytic ﬂ ow reactor: CO

oxidation on Pt(110). (Top) Mass spectrometer signals of O

, CO, and CO

measured at the output of

the ﬂ ow reactor cell. (Bottom) STM images on the Pt(110) catalyst surface acquired at selected stages

of the process: CO adsorption (A), O

ﬂ ow (B–D), and repetition of the sequence (E–H). Note the strong

surface roughening in (D), associated with increased CO

evolution. (Reprinted with permission from

1002 P. Sutter

the tip over that species and tracks its coordinates as it diffuses over

the substrate. In the atom-tracking mode, the STM spends all of its time

measuring the diffusion trajectory, which results in substantially

improved time-resolution compared to time-lapse STM movies.

Atom tracking STM has been used to measure the diffusion kinetics

of Si (Swartzentruber, 1996) and SiGe (Qin et al., 2000) dimers on

Si(001) above room temperatures, of water molecules on Pd(111) (Mitsui

et al., 2002), and of Pd atoms in a Pd/Cu(001) surface alloy (Grant

et al., 2001). The latter example is illustrated in Figure 15–24. Pd atoms

in the surface alloy are imaged as protrusions in STM. Locking the

STM tip onto individual Pd atoms, their diffusion pathway can be

tracked. Analyzing the residence time (the time between hops) and

jump length leads to the conclusion that there is no time correlation

between individual Pd diffusion events, and that the diffusion is medi-

ated by surface vacancies rather than Cu adatoms, i.e., involves rapidly

diffusing vacancies visiting Pd atoms in the surface layer. From

the temperature dependence of the Pd hop rate—determined from

temperature-dependent tracking experiments—the activation energy

of the overall process, in this case equal to the sum of the vacancy for-

mation energy and the energy barrier for lateral Pd-vacancy exchange,

can be measured.

In contrast to time-lapse STM, in which the tip is scanned rapidly

across a larger ﬁ eld of view, the tip maintains close contact with the

diffusing entity in atom tracking. Hence, the observed diffusion process

could be affected by tip–sample interactions. While this question has

to be studied on a case-by-case basis, at least one system [SiGe dimer

Figure 15–24. Diffusion kinetics of Pd atoms in the Pd/Cu(001) surface alloy.

(a) Site visitation map of an individual Pd atom, obtained by atom-tracking

STM at a temperature of 62°C. The square array marks the position of the

Cu(001) unit mesh. In this dataset the atom hopped 853 times in a time interval

of 5557 s. (b) Temperature dependence between 31 and 69°C of the average hop

rate of an incorporated Pd atom. The average residence time of Pd atoms

decreases from 145.3 to 5.0 s in this temperature range. The data follow an

Arrhenius form with an activation energy of 0.88 eV and measured prefactor

of 10

12.4±0.4

American Physical Society.)

Chapter 15 Scanning Tunneling Microscopy in Surface Science 1003

diffusion on Si(001)] has been identiﬁ ed in which the diffusion mecha-

nism depends on the sign of the electric ﬁ eld between tip and sample

(Sanders et al., 2003), indicating that the presence of the probe tip can

indeed affect the measurement.

4.3 Atom and Molecule Manipulation

Atom and molecule manipulation (Hla and Rieder, 2003) experiments

utilize the sharp STM probe tip and the ability to control it laterally

and vertically with picometer resolution to build and modify

nanometer-scale structures, typically in combination with their analy-

sis by STM imaging and tunneling spectroscopy. While conceptually

related to atom tracking, manipulation experiments are performed in

a different regime: the sample is cooled to temperatures low enough

that all thermal diffusion is frozen out, and the tip is typically brought

into close contact to induce strong tip–adatom/molecule interactions.

Since the ﬁ rst demonstration of atomic manipulation of Xe/Ni(110)

by Eigler and Schweizer (1990), manipulation experiments have

branched out considerably, and now encompass scenarios as diverse as

controlled chemical reactions of individual molecules (Lee and Ho,

1999), contacting single molecules with atomic metal wires (Nazin et

al., 2003), construction of mechanical logic gates from adsorbed mole-

cules (Heinrich et al., 2002), and control of excess charge on individual

atoms at insulator surfaces (Repp et al., 2004). While generally based

on some form of tip–sample interaction, atom or molecule manipula-

tion can involve a number of different physical mechanisms: close

proximity and short-range chemical interaction between a tip atom and

adatom to affect the potential landscape seen by the adatom on the

surface, vibrational excitation within adsorbed molecules or between

substrate and adatom, electric ﬁ eld, or direct charging by tunneling

electrons. Different patterns of adsorbate motion can be induced,

including lateral hopping between adsorption sites on the substrate,

vertical transfer between sample and tip, rotation, as well as the con-

trolled making and breaking of individual bonds. Substrate surfaces

with relatively low symmetry, e.g., (110) surface orientations or stepped

surfaces, are often used to establish one-dimensional diffusion path-

ways between substrate adsorption sites that help guide the manipula-

tion process.

Lateral manipulation is based on establishing close proximity

between an adatom and the STM tip to increase the interaction between

them. The approach process is controlled via the tunneling resistance,

which is lowered from typically several hundred MΩ during imaging

to values of the order of 100 kΩ for manipulation. Depending on the

particular system, the lateral force needed to move an adsorbate

between adjacent adsorption sites can be either repulsive or attractive,

giving rise to manipulation by pushing and pulling, respectively. The

manipulation process itself is monitored by measuring the tip height

(or z-piezo voltage) during the lateral motion of the tip, as illustrated

in Figure 15–25 for Pb/Cu(211) (pulling) and CO/Cu(211) (pushing)

(Meyer and Rieder, 1998).

1004 P. Sutter

The adsorbate motion in both pushing and pulling modes is strongly

inﬂ uenced by the preferred adsorption sites deﬁ ned by the substrate.

A sawtooth-like vertical motion of the tip accompanies each jump of

the adsorbate between neighboring adsorption sites (Figure 15–25a and

b). The origin of this tip motion is illustrated in Figure 15–25c for

the example of an attractive tip–adsorbate interaction. Following the

approach to the adsorbate, the tip is moved laterally. Initially, the adsor-

bate, held in the potential well of a substrate adsorption site, does not

follow. The tunneling gap thus increases, and the tip moves forward

to maintain a constant tunneling current (i). Ultimately, the interaction

with the tip induces a lateral jump of the adsorbate into a neighboring

potential minimum. The tunneling gap closes abruptly and the feed-

back loop causes the tip to retract.

The precision and complexity of structures achievable by lateral

manipulation is illustrated in Figure 15–26 (Heinrich et al., 2002). CO

molecules adsorbed in monomer, dimer, or trimer conﬁ gurations on

Cu(111) give rise to distinct contrast in constant current STM. While

monomers and dimers are stable at low temperature (5 K in this

example), there are three distinct conﬁ gurations for trimers: a stable

three-fold symmetric “close-packed” arrangement, and metastable

“straight-line” and “bent-line” (“chevron”) conﬁ gurations, which relax

with time constants of the order of seconds into the stable state. Complex

structures consisting of up to 545 CO molecules were built with atomic

precision such that any neighboring molecules were initially in stable

dimer conﬁ gurations, but could be changed to an unstable “chevron”

conﬁ guration with a place exchange of a single molecule in the vicinity.

In this way, molecular cascades, similar to toppling rows of standing

Figure 15–25. Mechanisms of lateral atom manipulation. (a) “Pulling” manip-

ulation via attractive interaction between a Pb atom on Cu(211) and the W tip.

(b) “Pushing” manipulation via repulsive interaction between a CO adsorbate

and the W tip. In both cases a sawtooth-like tip height proﬁ le with the peri-

odicity of the substrate adsorption sites is observed (Reprinted with permis-

Illustration of the tip and adsorbate motion during manipulation via attractive

interaction. (i) Lateral motion of the tip, accompanied by an approach toward

the substrate. (ii) Hopping of the adsorbate, followed by a sharp retraction of

the tip.

Chapter 15 Scanning Tunneling Microscopy in Surface Science 1005

dominoes, could be constructed. Mechanical computation was demon-

strated by molecular cascades set up as logic AND gates, two-input,

and three-input sorters.

Apart from lateral manipulation, vertical manipulation, i.e., transfer

of adsorbates between sample and tip, is possible. Vertical manipula-

tion of CO molecules, for example, has been used for controlled modi-

ﬁ cation of the STM tip and to induce single molecule reactions (Lee

and Ho, 1999). Vertical transfer is also important for moving adatoms

or molecules across obstactles such as step edges, if they cannot be

surmounted by controlled lateral hops. Manipulation mechanisms

other than direct chemical interaction between the STM tip and adsor-

bates have received increased interest recently. Notably the different

roles of vibrational excitations are actively investigated (Komeda et al.,

2002; Stroscio and Celotta, 2004). Inelastic tunneling can, for instance,

drive controlled rotations of adsorbed molecules (Stipe et al., 1998b).

An intriguing example of atomic manipulation based on inelastic tun-

neling, the controlled manipulation of excess charge on single Au

atoms, is illustrated in Figure 15–27 (Repp et al., 2004). Individual Au

Figure 15–26. Molecule cascades. (Left) (A) Conﬁ gurations of neighboring CO molecules on Cu(111):

isolated CO molecule, dimer, and trimer in a “chevron” conﬁ guration. Large circles mark the positions

of the molecules and small dots indicate the surface layer Cu atoms (1.9 nm scans; T = 5 K). (B) The

same area after one CO molecule in the trimer has hopped to generate a stable close-packed trimer.

(Right) Demonstration of a molecular mechanical AND gate, implemented via cascades of “chevron”-

type CO trimers. (A) Model of the AND gate. (B–D) Sequence of STM images (5.1 nm by 3.4 nm)

showing the operation of the AND gate: (B) Initialization. (C) Result after input X was triggered with

the STM tip. (D) When input Y was triggered, the cascade propagated all the way to the output. (E)

1006 P. Sutter

atoms are deposited on ultrathin, insulating NaCl supported by Cu(111).

Electrons can tunnel between the metal substrate and the STM tip

through the ultrathin insulator, i.e., STM imaging, spectroscopy, and

manipulation are possible for this system. However, the coupling of

the Au adsorbate to the substrate is affected profoundly by the insulat-

ing support. Voltage pulses can be used as a means to reversibly deposit

an excess electron on individual Au atoms. The charging occurs via an

inelastic electron tunneling mechanism enabled by a weak coupling of

the adatom and Cu substrate electronic states. The data suggest that

the coupling is so weak that the lifetime of a negative ion resonance

state of the adatom is in the range of ionic vibrational periods for the

NaCl interlayer, allowing the relaxation of the NaCl lattice, shift of the

negative ion resonance state below the Fermi energy, and capture of

the electron. The excess charge is maintained due to the substantial

relaxation of the underlying NaCl lattice. This stabilization prevents

discharging by tunneling into the metal, and the electron resides on

the Au atoms until removed by a voltage pulse of opposite sign. The

charged Au atom has a distinct signature in constant-current STM

imaging, and the long-range electric ﬁ eld due to the charged Au atom

strongly scatters electrons in interface states at the Cu/NaCl interface.

Charging individual adsorbed atoms is a potentially powerful approach

to tuning their physical properties. For Au/NaCl, signiﬁ cant differ-

ences in surface diffusivity were identiﬁ ed, with the onset temperature

of signiﬁ cant surface diffusion lowered from 60 K (Au) to 40 K (Au

−

Other possible modiﬁ cations due to controlled charging at the atomic

level include changes in catalytic activity and the controlled manipula-

tion of the net spin magnetic moment of individual atoms.

Figure 15–27. Charging of individual Au atoms on ultrathin NaCl/Cu(111). (a) Constant-current STM

on a pair of Au atoms, one of which was charged negatively with the other remaining neutral. (b)

Signature of the charging event. Successful charging, achieved by a voltage pulse while maintaining

the tip above a single Au atom, is indicated by a sudden drop in tunneling current. (c and d) Quasi-

particle scattering: neutral Au adatoms do not scatter NaCl/Cu(111) interface-state electrons (c),

whereas the negatively charged adatom acts as a scatterer (d). (Reprinted with permission from Repp

Chapter 15 Scanning Tunneling Microscopy in Surface Science 1007

5 Heterostructures and Buried Interfaces: BEEM,

Quantum Size Effects, and Cross-Sectional STM

The exponential dependence of the tunneling current on the tip–sample

separation makes STM a versatile tool for atomic-resolution micros-

copy and spectroscopy at surfaces. As a result of its unique resolution

and surface sensitivity STM has brought important advances in ﬁ elds

such as catalyis or epitaxial growth, in which surface processes play a

key role. For many technological applications of semiconductor materi-

als, e.g., in device structures such as transistors, detectors, or lasers, the

active regions encompass complex heterostructures, and device per-

formance is affected much less by the free surface than by buried

interfaces. Therefore, a need arises for a technique capable of probing

subsurface structures, electronic properties, and carrier transport. The

mapping of interfacial and transport properties should occur with

nanometer spatial resolution to provide data that are relevant for semi-

conductor devices with progressively reduced dimensions. Since the

early 1990s, several STM-based techniques have been developed to

provide high-resolution imaging and spectroscopy of buried interfaces

and heterostructures. Ballistic electron emission microscopy (BEEM)

operates in a three-terminal conﬁ guration, in which the STM tip, whose

height is controlled by a constant-current feedback loop, injects hot

carriers into a thin ﬁ lm or heterostructure while an additional collector

contact measures the current of carriers that are transmitted through

buried interfaces. Electron interference in a thin metal ﬁ lm, again

with carriers injected from an STM tip and traveling ballistically in

the metal, provides detailed thickness maps of metallic overlayers,

and can—under favorable circumstances—even map the atomic struc-

ture of a buried metal–semiconductor interface. Cross-sectional STM,

ﬁ nally, is used to image complex heterostructures such as superlat-

tices, embedded self-assembled quantum dots, or substitutional mag-

netic impurities, on nonpolar (110) cleavage planes of III–V compound

semiconductors.

5.1 Probing Buried Interfaces (I)—BEEM

BEEM was invented by Kaiser and Bell (1988) to probe Schottky con-

tacts, i.e., metal–semiconductor interfaces with high spatial resolution

(for a recent review, see Narayanamurti and Kozhevnikov, 2001).

Beyond Schottky barriers, the technique can determine the height of

other subsurface potential barriers and it has also been applied to

measure band offsets between different semiconductors, the energies

of transmission resonances in semiconductor quantum wells and

superlattices, and bound states in buried quantum wires and dots. In

addition, electron scattering at subsurface linear and point defects has

been characterized.

We illustrate the operating principle of BEEM using the example of

a metal–semiconductor junction. The technique operates in a three-

terminal conﬁ guration, i.e., adds an additional collector electrode to the

STM tip and sample contact, as shown schematically in Figure 15–28.

1008 P. Sutter

The STM tip is scanned at constant current I

, measured between the

tip and base electrode, over the surface of the base layer. The tip serves

as a point source of hot carriers, electrons or holes, injected into the

metal base. If the metal thickness is small compared to the mean free

path for electron–phonon, electron–electron, or impurity scattering,

most carriers reach the metal–semiconductor interface ballistically, i.e.,

with their original energy and momentum distribution. In metals,

the mean free path for electron–phonon scattering of electrons with

energies of a few eV is typically of the order of 10 nm. In signiﬁ cantly

thicker ﬁ lms, electron–phonon scattering would broaden the momen-

tum distribution of the injected carriers, i.e., mostly affect the spatial

resolution of BEEM, while having little effect on the energy distribu-

tion. Hence, the injected ballistic carriers generally have a well-deﬁ ned

energy distribution and can be used to perform spectroscopic mea-

surements on buried potential barriers.

At the metal–semiconductor interface, ballistic carriers with energies

below a threshold equal to the Schottky barrier height eφ

are reﬂ ected

back into the metal base, while carriers with energy above the thresh-

old are transmitted into the semiconductor collector (Figure 15–29). If

Figure 15–28. Schematic setup

of a typical BEEM experiment

on a metal (M)–semiconductor

(S) heterostructure.

Figure 15–29. Band diagrams for (a)

ballistic electron and (b) ballistic hole

injection into the metal base on an

n- and p-type semiconductor, respec-

tively. The dark shaded areas indicate

the energy distribution of the injected

carriers, as well as of those transmitted

into the collector. The inset in (a) illus-

trates the onset of the collector current

at a threshold bias corresponding to

the Schottky barrier height, eφ

. (After

Bell et al., 1991.)

Chapter 15 Scanning Tunneling Microscopy in Surface Science 1009

other barriers exist in the collector layer, the transport through that

material can involve additional interfacial reﬂ ections. Carriers that are

transmitted through the entire collector layer contribute to the collector

current (or BEEM current), which is measured between base and col-

lector contacts.

The energy of the injected carriers is varied by changing the voltage

applied between the STM tip and metal base. For low bias, none of the

injected carriers is transferred across the metal–semiconductor inter-

face. At voltages above the threshold, some carriers have energy above

the Schottky barrier and can cross the interface, causing an increase in

collector current with increasing tip–sample bias above threshold.

From the onset of the BEEM current, the barrier height eφ

can be

determined, e.g., by ﬁ tting the measured I

(V) to a theoretical expres-

sion. In a simple one-dimensional theory,

IV RI dEfE fE eV E E eV e

ct F B

(

)

(

)

−−

(

)

[]

×−−+

(

)

[]

∫

θφ (6)

where R is a bias independent parameter and f(E) is the Fermi

function.

Figure 15–30 shows experimental BEEM spectra on an Au/n-Si(001)

junction, measured at room temperature in N

atmosphere, ﬁ tted using

Eq. (6) to determine a Schottky barrier eφ

= 0.92 eV. Due to its low

reactivity and oxidation resistance, Au was used in many of the early

BEEM experiments as a nonepitaxial metal base on a variety of other

semiconductors. Schottky barrier heights were determined, for example,

for Au/n-GaAs(001) (eφ

= 0.70 eV at 77 K) (Bell et al., 1990) and Au/n-

GaP(110) (eφ

= 1.41 eV) (Prietsch and Ludeke, 1991). For Au/n-CdTe(001)

(eφ

= 0.69–1.07 eV) (Fowell et al., 1990), strong lateral variations in the

Figure 15–30. BEEM spectra obtained on a polycrystalline Au/n-Si(001) junc-

tion. Spectra a–c (symbols) are measured at different tunneling currents. The

calculated spectra (solid lines) correspond to a common Schottky barrier

height value φ

= 0.92 eV and R value of 0.045 eV

−1

. (Reprinted with permission