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990 P. Sutter

measured simultaneously with dI/dV maps at one or several voltages/

energies. To obtain the spectroscopic data, the feedback loop is turned

off at each point of the STM scan, and the selected bias voltages are

applied. The tunneling conductance is measured directly using a mod-

ulation technique. At the end of the measurement, the feedback is

reactivated, and the tip is moved to the next image pixel.

If a small modulation voltage dV(t) = a cos ωt with amplitude a and

frequency ω is added to the dc sample bias V

, the resulting tunneling

current

It I dIt I

dV t

(

)

(

)

(

)



ssωt + 

(5)

has a small ac component. Its amplitude at the modulation frequency

ω, which can be measured using a lock-in ampliﬁ er, is proportional to

dI/dV|

. Via measurements at different dc bias V

, a set of tunneling

conductance values at different energies can be determined at each

pixel during an STM scan, and can later be displayed as maps of the

sample density of states at those energies. Although quite simple con-

ceptually, the long dwell times at each image pixel in dI/dV mapping

pose stringent demands on microscope stability, which are best met at

cryogenic temperatures. Under these conditions, thermal broadening

of features in the density of states is also minimized.

dI/dV mapping has been applied in a wide variety of measurements

of electronic states at surfaces and in nanostructures, such as atomic

Figure 15–16. I(V) tunneling spectroscopy on C

/C-nanotube “peapods” (Reprinted with permission

nanotube “peapod.” Sample bias voltage is plotted on the horizontal axis and displacement along the

tube on the vertical axis. (B) Representative dI/dV spectra at selected positions along the tube. Large

conductance peaks are found at positions of embedded C

molecules. (C) Variation of tunneling con-

ductance along the tube axis. (D) Reference spectroscopic map on an empty C-nanotube section

without embedded C

, in which no strong modulation of the tunneling conductance is observed. (See

color plate.)

Chapter 15 Scanning Tunneling Microscopy in Surface Science 991

chains (Crain and Pierce, 2005) or single molecules (Repp et al., 2005).

A particularly widespread application is the imaging of quasiparticle

interference. In an ideal metal, the Landau-quasiparticle eigenstates are

Bloch wavefunctions with speciﬁ c wavevectors k and energies ε. Their

dispersion cannot be measured directly by STM. However, in the pres-

ence of disorder, e.g., impurities or crystal defects, elastic scattering

mixes eigenstates with different k that are located on the same quasi-

particle contour of constant energy in k-space. When states k

and k

are mixed by scattering, an interference pattern with wavevector q =

− k

appears in the norm of the quasiparticle wavefunction. The

interference leads to modulations in the local density of states with

wavelength λ = 2π/|q|, wh ic h can be observed as modulations in the

differential tunneling conductance. Via differential conductance

mapping, interference of electronic eigenstates has been probed in

metals (e.g., Cu(111), Crommie et al., 1993; Au(111), Hasegawa and

Avouris, 1993) and semiconductors (e.g., InAs(110), Wittneven et al.,

1998). When density of states modulations are detected by STM, certain

contours of constant energy in k-space can be reconstructed by analyz-

ing the Fourier transform of the real-space density of states map. Such

Fourier transform spectroscopy simultaneously yields real-space and

momentum-space information on wavefunctions, scattering processes,

and quasiparticle dispersion.

Elastic scattering of Bogoliubov quasiparticles in superconductors

can also give rise to conductance modulations (Hoffman et al., 2002a),

suggesting that STM could be used as a local probe to study electron

correlation in superconductors. Quasiparticle inteference has indeed

been demonstrated for cuprate high-temperature superconductors, such

as Bi

CaCu

8+δ

(Hoffman et al., 2002a; Vershinin et al., 2004) and

2−x

CuO

(Hanaguri et al., 2004). On Bi

CaCu

8+δ

crystals

cleaved at the BiO plane at cryogenic temperature in UHV, a constant-

current image (Figure 15–17A) and differential conductance maps

(Figure 15–17B) are obtained simultaneously in a low-temperature

Figure 15–17. Fourier transform spectroscopy on a cleaved Bi

CaCu

8+δ

high-temperature super-

conductor. (A) Constant-current STM image (65 nm, 0.13 nm resolution). (B) Simultaneously acquired

differential conductance map (∼ local DOS) at an energy of 12 meV below E

, showing a pronounced

modulation of the electronic structure. dI/dV is measured with 2 mV modulation. (C) Fourier trans-

forms of DOS maps similar to (B), for energies up to 30 meV, representing maps of quasiparticle scat-

AAAS.)

992 P. Sutter

STM. The conductance map shows a checkerboard-like modulation of

the local density of states. Fourier transforms of dI/dV maps obtained

over a range of energies show dominant q-vectors associated with

quasiparticle scattering. Different spatial patterns and wavelengths are

observed at different energies. Applying a magnetic ﬁ eld perpendicu-

lar to the sample, the quasiparticle scattering can be modiﬁ ed by the

controlled introduction of vortices (Hoffman et al., 2002b). Although

the interpretation of these complex data sets is still somewhat contro-

versial (Vershinin et al., 2004), these experiments clearly demonstrate

the power of an STM-based approach for studying electronic structure

and ordering phenomena in high-temperature superconductors.

A second example in which dI/dV mapping plays a key role is spin-

polarized STM. A magnetic probe tip, e.g., a conventional etched W tip

coated with a thin Fe ﬁ lm and polarized in a magnetic ﬁ lm perpen-

dicular to the tip axis, is used for tunneling on a magnetic sample.

Figure 15–18 illustrates the principle of spin-polarized STM for the

example of a ferromagnetic Gd(0001) sample and a hard magnetic Fe

tip (Bode et al., 1998). In a weak external magnetic ﬁ eld applied parallel

to the sample surface, Gd(0001) has an exchange-split surface state

with majority and minority parts at binding energies of −220 meV and

+500 meV, respectively (Figure 15–18a). In the experiment, the magne-

tization direction of the Fe tip remains ﬁ xed independent of the direc-

tion of the applied magnetic ﬁ eld, while the magnetization of the Gd

sample can be switched by reversing the direction of the applied ﬁ eld.

Due to a spin valve effect, the tunneling current of the surface state

spin component parallel to the tip magnetization is enhanced. If the

majority spin is aligned with the tip magnetization (blue curve in

Figure 15–18b) the tunneling conductance of the occupied majority

Figure 15–18. Spin-polarized STM on a Gd(0001) sample with an exchange-split surface state and a

magnetic Fe tip with constant spin polarization close to E

. (a) Due to the spin-valve effect the tun-

neling current of the surface state spin component parallel to the tip magnetization is enhanced. (b)

Illustration of the reversal in the dI/dV signal at the surface state peak position upon switching the

sample magnetically. (c) Experimental observation of this reversal in tunneling into an isolated Gd

color plate.)

Chapter 15 Scanning Tunneling Microscopy in Surface Science 993

state will be high, while that of the minority state is low. After reversal

of the applied magnetic ﬁ eld, the minority spin will align with the tip

magnetization. The conductance of the minority state thus increases,

while that of the majority state is reduced. Difference spectra obtained

by subtracting the traces, measured at two polarities of the external

ﬁ eld, provide a direct measure of the sample magnetization, and can

be used to build maps of magnetic order. Spin polarized STM has been

shown to be a very powerful technique for mapping magnetic and

antiferrromagnetic order with atomic resolution, as demonstrated by

studies on 2D antiferromagnetic ordering in Mn/W(110) (Heinze et al.,

2000) and on magnetic hysteresis in Fe/W(110) (Pietzsch et al., 2001).

3.4 Inelastic Tunneling: Vibrational Spectroscopy

A major drawback of conventional STM imaging is its lack of chemical

and element speciﬁ city, which stems from the fact that only states

within few eV of the Fermi energy can be probed. As a result, charac-

teristic core levels, which could provide element speciﬁ city, are not

accessible. For single molecules, an alternative pathway to chemically

speciﬁ c imaging employs vibrational signatures, which are character-

istic of speciﬁ c molecular bonds.

The use of inelastic electron tunneling spectroscopy (IETS) for vibra-

tional ﬁ ngerprinting on molecules embedded in planar tunnel junc-

tions dates back to the 1960s (Jaklevic and Lambe, 1966). The technique

is based on the detection of a slight increase in tunneling conductance

at an electron energy eV =

hω, corresponding to a vibrational mode

with frequency ω of an embedded molecule. When the applied bias

reaches this threshold, an inelastic channel for tunneling opens up in

addition to the elastic tunneling at lower bias. Inelastically scattered

electrons make up only a small fraction of the total tunneling current

(typically a few percent). Nevertheless, the second derivative of the

tunneling current, (d

I/dV

), usually shows clear signatures of inelastic

tunneling in the form of peaks at the characteristic bias voltages cor-

responding to different vibrational modes. As in dI/dV maps, d

I/dV

can be measured by lock-in techniques, by adding a small modulation

voltage to the tunneling bias V

. The signal amplitude measured at

twice the modulation frequency is proportional to d

I/dV

Conventional IETS in planar oxide tunnel junctions samples large

ensembles of embedded molecules. Soon after the invention of STM, it

was suggested that IETS should be feasible in local tunneling between

a tip and a sample in STM (Binnig et al., 1985). A ﬁ rst convincing

experimental demonstration of inelastic tunneling through few mole-

cules was obtained in a setup of two crossed Au wires with ﬁ nely

adjustable normal force, allowing the trapping and IET spectroscopy

at cryogenic temperatures on a small number of hydrocarbons

(Gregory, 1990).

IETS on single molecules in a low-temperature STM was ﬁ nally

demonstrated by Ho and co-workers in 1998 (Stipe et al., 1998a). A

beautiful experiment on single acetylene molecules adsorbed on

Cu(100) not only provides clear evidence for inelastic tunneling at

994 P. Sutter

energies close to vibrational frequencies determined in the gas phase,

but it also shows the expected isotope shift between measurements on

normal and deuterated acetylene. Robust increases in differential con-

ductance of the order of 3–12% are observed for this system.

In addition to inelastic tunneling spectra on individual molecular

bonds, as shown in Figure 15–19a, inelastic electron tunneling micros-

copy with molecular resolution was demonstrated on small molecules,

such as acetylene (Figure 15–19b). In constant-current STM imaging,

both C

and C

molecules are imaged as identical depressions, i.e.,

are indistinguishable. On the other hand, C

molecules are selec-

tively imaged in (d

I/dV

) maps obtained at the characteristic energy

of the C—H stretch mode (358 meV), while C

molecules are imaged

at the isotope shifted energy (266 meV). In a control experiment at an

energy between these two characteristic values, none of the two iso-

topes gives rise to contrast.

The development of single-molecule vibrational spectroscopy and

microscopy provides a powerful tool for performing and viewing

chemistry at the ultimate limit of concentration (individual molecules)

and spatial resolution (atomic). Local energy input by inelastic tunnel-

ing allows the precise manipulation of individual molecules (rotation,

translation, see Section 4) and the cleavage and formation of individual

molecular bonds, as well as the characterization of the individual and

ﬁ nal conﬁ gurations on the surface.

Figure 15–19. Single molecule vibrational spectroscopy and microscopy. (a) d

I/dV

recorded over

individual C

and C

molecules on Cu(100) at 8 K. (1) Signature of inelastic electron tunneling by

exciting the C—H stretch mode with an energy of 358 meV. (2) Isotope shifted (266 meV) inelastic tun-

neling peak of the C—D stretch mode. (b) Constant current (CC, 4.8 nm) STM of adjacent C

and

molecules on Cu(100), along with simultaneously acquired d

I/dV

maps obtained at 358 mV and

266 mV, imaging the C—H and C—D bonds selectively. The symmetric round appearance is attributed

to rotations of the molecules during the experiment. A d

I/dV

map obtained at intermediate energy

Chapter 15 Scanning Tunneling Microscopy in Surface Science 995

3.5 Element-Speciﬁ c Imaging

In contrast to chemical speciﬁ city at the level of individual small mol-

ecules, which IETS can provide independent of the particular sample

structure, no universal approach has been identiﬁ ed to obtain element

speciﬁ c contrast in STM. Various techniques have been suggested to

achieve generic element speciﬁ city, among them a scanning tunneling

atom probe operated by controlled atom transfer to the STM tip and

desorption into a time-of-ﬂ ight chamber (Weierstall and Spence, 1998),

and a scanning probe energy loss spectrometer using an STM at high

tip–sample bias in combination with an electron spectrometer for

local probe electron energy loss spectrometry (Festy and Palmer, 2004).

None of the techniques demonstrated to date provided a universal and

practical pathway toward element speciﬁ c imaging at the atomic scale.

However, a number of sample–tip combinations have been identiﬁ ed,

in which element contrast is indeed achieved on a case-by-case basis.

Prime examples are bimetallic metal surfaces, on which different

atomic species can be discriminated in constant current STM images.

In a few simple cases, a mere difference in atomic size can give rise

to contrast, e.g., for submonolayer coverages of Pb/Cu(111) with a dif-

ference in metallic radii of nearly 50 pm (Nagl et al., 1994). In a broader

class of metal alloys, electronic effects, i.e., differences in local density

of states above different atoms, give rise to element contrast. Examples

of systems with substantial density of states differences near the Fermi

energy include alloys between transition metals with a partially ﬁ lled

d-shell and noble metals (Au, Ag, Cu). STM images of the (111) surface

of AgPd alloys (Wouda et al., 1998) indeed show the Ag atoms with

lower apparent height than the transition metal atoms, as expected

based on a simple density of states argument. For alloys of transition

metals (e.g., PtRh; Wouda et al., 1996), the prediction of the STM con-

trast is less straightforward, and generally requires ab initio calcula-

tions of the local density of states. For the PtRh(100) surface shown in

Figure 15–20a, a local density of states difference at low tunneling

resistance produces a height difference of about 20 pm between Pt and

Rh, with Rh imaged at larger apparent height. In some systems, where

sufﬁ cient differences in local density of states to provide STM contrast

do not exist, such differences can be induced by suitable modiﬁ cation

of the sample surface. In an important semiconductor system, Si

1−x

alloys, Si and Ge atoms are indistinguishable in conventional constant

current STM, which severely hampers the understanding of epitaxial

growth since crucial local composition information is lacking. Cover-

ing a heterogeneous (111) surface composed of Si- and Ge-rich areas

with 1 atomic layer of Bi induces robust differences in apparent height

of nearly 0.1 nm between the chemically different regions, thus provid-

ing a means for imaging and characterizing lateral heterostructures

of monolayer-thick alternating epitaxial Si and Ge stripes (Kawamura

et al., 2003).

A third mechanism that can give rise to atomic-scale contrast between

different elements is thought to be based on a direct interaction between

tip and sample, in what could be described as a “precursor” chemical

996 P. Sutter

bond. With the tip–sample distance too large to allow true chemical

bonding, either atomic relaxation or the local charge density can change

as the tip is moved across different atomic species, which in turn

affects the tunneling current and gives rise to apparent height differ-

ences in constant current STM. Qualitatively, surface atoms with

higher chemical afﬁ nity to the tip atom should have higher tunneling

conductance and thus will be imaged at larger apparent height. Several

metal alloy systems, such as PtNi(111) and PtRh(111), are believed

to show element contrast that is based on tip–sample interaction.

Clearly, the nature of the tip apex would be of key importance for this

type of contrast. This may explain why for some systems element

contrast is obtained only after deliberate chemical modiﬁ cation of the

STM tip.

4 STM at High and Low Temperatures

STM combines unique capabilities in both high-resolution imaging and

spectroscopy at surfaces. Some form of controlled environment—such

as UHV, an inert gas or ﬂ uid—may be required, mainly to ensure

reproducible sample surface conditions. But many basic microscopy

tasks, including a broad range of imaging and spectroscopy experi-

ments, are performed conveniently at room temperature. Soon after the

invention of STM, however, it was recognized that control over the

sample temperature enables a wide variety of new experiments that

are impossible to perform at room temperature. As an example, the

structure or chemical reactivity of a given surface, e.g., of a catalyst,

may depend on temperature. To characterize its properties in a particu-

lar temperature regime, the sample has to be heated or cooled during

Figure 15–20. Element-speciﬁ c imaging with atomic resolution by STM. (a)

PtRh(100): for a bulk composition of 50% Pt and Rh, there are approximately

31% Rh (bright) and 69% Pt atoms (darker) at the surface. Black spots are

caused by residual carbon. Pt and Rh atoms tend to cluster in small groups of

permission from Elsevier). (b) Atomically and chemically resolved image of

Ge (bright)/Si (dark) nanowires formed by sequential deposition of submono-

layer amounts of Ge and Si in step ﬂ ow mode on Bi-terminated Si(111). The

initial Si/Ge boundary (right arrows) is nearly atomically sharp, while inter-

diffusion is observed at the Ge/Si boundary (arrowheads) (Reprinted with

Chapter 15 Scanning Tunneling Microscopy in Surface Science 997

STM imaging. Further, though imaging of thin ﬁ lm growth in a “quench

and look” mode is possible, as discussed previously, studying growth

processes directly at relevant sample temperatures would often be

preferred. And ﬁ nally, investigation of surface diffusion, chemical

reactions, or molecular dissociation at surfaces often requires cooling

of the sample to cryogenic temperatures to slow the rates of these

thermally activated processes sufﬁ ciently to map them by relatively

slow STM imaging. Probably the best known example of the beneﬁ ts

of cryogenic STM is atom and molecule immobilization for atomic or

molecular manipulation, which allows artiﬁ cial structures to be built

from individual atoms and can provide highly idealized structures for

probing chemistry and condensed matter physics at the ultimate spa-

tial limit. Below, the technical challenges involved in temperature-

dependent STM are discussed brieﬂ y, and selected examples of the

main classes of STM experiments at high and low temperature—the

imaging of dynamic surface processes via STM movies and the assem-

bly of nanostructures “one atom at a time” by atomic/molecular

manipulation with the STM tip—are presented.

4.1 STM at High and Low Temperatures: Motivation and Issues

Compared to room temperature operation, STM experiments at high,

low, or even variable temperatures involve a number of additional

issues. A primary experimental difﬁ culty arises from thermal drift due

to incomplete thermalization and the resulting variations in thermal

expansion across the microscope, which cause relative motion between

the STM tip and sample. Thermal expansion coefﬁ cients are tempera-

ture dependent, and generally approach zero as T → 0. As a result,

thermal drift rates tend to be low and the overall instrument stability

is improved in low-temperature STM. Stable low-temperature micro-

scopes reach lateral drift rates of 0.1 nm/h (Meyer and Rieder, 1997),

and values of vertical stability of the tunneling gap approaching 1 pm

have been reported (Stipe et al., 1998a). Beyond instrument stability,

there are strong additional incentives for operation at cryogenic tem-

peratures. The rates of thermally activated processes, such as surface

diffusion of adsorbed atoms or molecules, can be slowed considerably

at low temperatures. The freeze-out of thermal motion paves the way

for time-lapse movies documenting diffusion processes, and for con-

trolled atom or molecule manipulation using the STM tip. On the other

hand, the thermal broadening of the tunneling distribution and of the

electronic structure can be minimized at cryogenic temperatures,

which results in higher energy resolution in spectroscopy.

STM imaging at high temperatures is generally affected by high drift

rates, which may require active drift compensation between consecu-

tive scans to allow imaging a given region on the sample for extended

periods of time. Intrinsic drift rates of 3 nm/min without and 0.2 nm/

min with drift correction have been reported for dedicated high-

temperature microscopes (Voigtländer, 2001). Additional practical dif-

ﬁ culties involve sample heating, temperature measurement, as well as

the need for high scan speeds to capture dynamic phenomena at high

temperatures.

998 P. Sutter

4.2 Capturing Dynamic Surface Processes: STM Movies and

Atom Tracking

The most popular approach for imaging dynamic processes involves

repeated scanning of the same sample area to generate a time-lapse

STM “movie.” This capability has been used to study a wide variety of

phenomena, including step dynamics (Kuipers et al., 1993), epitaxial

growth (Voigtländer and Zinner, 1993), self-diffusion on semiconduc-

tor (Borovski et al., 1997) and metal surfaces (Horch et al., 1999), as well

as diffusion of adsorbates (Wintterlin et al., 1997) and large molecules

(Schunack et al., 2002). If rates of dynamic processes, such as surface

diffusion, are measured at several temperatures, activation energies of

these processes can be determined. Complex reaction pathways,

in cluding adsorption, diffusion, dissociation, etc., can be elucidated at

the atomic scale, and active sites can be identiﬁ ed.

As a near-ﬁ eld microscopy technique STM is based on raster scan-

ning a probe tip. The scan speed is limited by the resonance frequency

of the scanning element, typically below about 10 kHz, and by the

electronic bandwidths of the tunneling current ampliﬁ er and feedback

circuit. Frame rates in STM movies can be expected to be inherently

lower than those in parallel imaging techniques, such as transmission

electron microscopy or low-energy electron microscopy (Bauer, 1998;

Tromp, 2000). To achieve adequate time resolution, the scan sizes in

STM movies tend to be small, of the order of 10

pixels. Additional

measures are typically necessary to deal with slow imaging rates and

capture the detailed time evolution of a given process. Whenever a

phenomenon under consideration permits, cooling of the substrate can

be employed to slow the rates of thermally activated processes. In thin

ﬁ lm growth, slow evaporation rates are employed. In studying surface

reactions involving adsorption from the gas phase, e.g., on catalysts,

low partial pressures of the reactive species in the gas phase are main-

tained. As an alternative to these often rather restrictive provisions, the

development of high-speed microscopes and control electronics has

been a focus of active research recently. As an example, scanners with

mechanical resonances in the range between 50 and 100 kHz have been

developed. Combined with a fast current ampliﬁ er (600 kHz band-

width) and feedback loop (1 MHz), atomically resolved images have

been obtained at rates approaching 200 frames/s (Rost et al., 2005).

Such exciting instrument developments may in the future allow the

imaging of new classes of dynamic surface phenomena at relaxed

ambient conditions (higher temperatures, pressure, and growth rates)

by STM.

To illustrate the use of STM movies at cryogenic temperatures to

identify and quantify dynamic surface processes, we discuss the

example of rutile TiO

(110), which has emerged as the prototypical

system for fundamental surface science studies of transition metal

oxides (Figure 15–21). TiO

has numerous applications in areas as

diverse as heterogeneous catalysis, solar cells, photocatalysis, and

organic waste remediation, and STM plays a key role in elucidating its

fundamental surface processes. The TiO

(110)-(1 × 1) surface consists

Chapter 15 Scanning Tunneling Microscopy in Surface Science 999

of alternating rows of Ti and O atoms aligned along the [001] direction.

Due to electronic effects, STM images Ti as protruding rows and bridg-

ing oxygen rows, which are geometrically highest on the surface by

about 0.12 nm, as troughs.

The reactivity of TiO

(110) is affected to a great extent by the presence

of oxygen vacancies, which are generated in the process of reducing

the surface by annealing in vacuum. Using STM movies, the reaction

mechanisms at these defect sites were studied for model reactions such

as water dissociation (Brookes et al., 2001; Schaub et al., 2001). The dif-

fusion of oxygen vacancies follows an intriguing mechanism mediated

by O

molecules (Figure 15–22) (Schaub et al., 2003). Oxygen vacancies

Figure 15–21. STM on Rutile TiO

(110). (a) Structure of the TiO

(110) surface. V, a single oxygen

vacancy. (b) Constant-current STM on TiO

(110). Bright rows are assigned to ﬁ ve-fold coordinated Ti

atoms and dark rows to bridging oxygen atoms. Oxygen vacancies are imaged as protrusions between

the Ti rows. Adsorbed O

molecules are associated with bright protrusions on the Ti rows. (Reprinted

Figure 15–22. O

-mediated diffusion of oxygen vacancies on TiO

(110). (a and b) Consecutive frames

of an STM movie on the motion of oxygen vacancies on TiO

(110) (T = 300 K, 8.5 s/frame). (c) Difference

image of (a) and (b), highlighting changes due to the diffusion of single oxygen vacancies. Vacancies

diffuse perpendicular to the bridging oxygen rows. (d–f) Time-lapse STM of the O

-assisted diffusion

of an individual oxygen vacancy (T = 230 K, 1.1 s/frame). (g) Schematic illustration of the O

-mediated