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Superlattices Thin Films and Materials DesignЧA Mechanistic Approach.

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cence detection techniques." 21 With that in mind, this combination triggers intriguing possibilities for chemists and biochemists: Single molecules can conceivably be separated, collected. and manipulated on a chip, and possibly even made to
undergo chemical reactions. Chemistry with single molecules
could soon be reality.
German version: Angeu Chern. 1996, 108, 931 --933
Keywords: analytical methods
capillary electrophoresis
[ l ] A selection o f r n i e w articles and monographs: J. W Jorgenson. K. D. Lukacs,
Scien<r 1983, 322. 266-272; M. J. Gordon. X . Huang, S. L. Pentoney, Jr..
R. N Zare. f h f d 1988. 242, 224-228; A. G Ewing, R. A. Wdllingford. T. M.
Olefirowicz. A n d Cizrm. A 1989. 61. 292-303: H. Engelhardt, W.Beck, J
Kohr. T Schmiit. Angcii.. C'keni. 1993, 105. 659-680: Angeir. Cheni. Inr. Ed.
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Lihr 53) Elsckier, Amsterdam 1992, Cupillurr Ele( Tedinolog~
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(Ed. : J. P Landers) CRC Press, Boca
Honrll~ooko/ C'upillur>.
Raton. FL. USA. 1993
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[5] K. Albert. A n g e u . Chem. 1995, 1117, 699-701: Angeir. Cliem. Inr Ed. Engl.
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[6] D.L. Olson. T. L. Peck, A. G . Webb. R L. Mangin, J. V Sweedler. Scicwce
1995, 270. 1967-1969.
[7] Y. M. Liu. J. V. Sweedler, J Am. Chem Soc. 1995. 117. 8x71 -8872.
[8] D. J. Harrison, K. Fluri, K. Seiler, Z. Fan, C. S. Effenhauser. A. Manz, Science
1993,261, 895-897, C. S . Effenhauser. A. Manz, H. M. Widmer, Anul. Chrni.
1993.65. 2637-2642
[9] C. S. Effenhauser. A. Paulus, A. Manz, H. M. Widmer. A n d C'hern. 1994, 66,
[lo] S. C. Jacobson, R. Hergenroeder. A. W Moore. Jr., J. M. Ramsey, A n d . C/iem.
1994. 66, 41 27 -41 32
[11] C.S Effenhauser. A. Manz, H. M. Widmer. A n d Clitwr 1995, 67, 22842287.
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Superlattices, Thin Films, and Materials Design-A
Mechanistic Approach
Robert Schollhorn*
The area of thin films is rapidly developing into a key technology with applications ranging from electronic materials (semiconductors, superconductors, optoelectronics etc.) to chemical
sensor systems. A variety of deposition techniques for the preparation of single layers and superlattices is now available, extending from molecular beam epitaxy to chemical vapor deposition.
While physicists are focussing their attention on the detection of
new quantum phenomena associated with the constrained morphology in nanorange dimensions, it appears that a new avenue
is opening here for solid-state chemistry through the controlled
synthesis of complex metastable structures. A particularly interesting example in this line is represented by the approach documented in recent work by D. C. Johnson and coworkers which
concerns the preparation of new compositionally modulated
layered transition metal dichalcogenide structures characterized
by the use of a rational synthesis strategy that involves a topological model for kinetic trapping and directed interfacial
growth." --'I
Superlattices are defined here as multilayer thin films with
long-range structural coherence perpendicular to the basal
plane which are prepared by time sequential deposition of components. In the area of molecular solids the formation of ordered multilayer systems with complex arrays of subunits has
become an established art (for example, Langmuir-Blodgett
Prof. I h . R. Schbllhorn
lnstitut fur Anorganische und Analytische Chemie, Sekr. C 2
Technische Universitit Berlin
Strssae des 17 J u n i 135. D-10623 Berlin (Germany)
Fax: Int. code +(30) 314-21106
e-mail: schoe(
lrir E d D i g / . 1996. 35. No 8
and polymer films).14* The deposition of multilayers of metals/
alloys,[6] covalent semiconductor materials,['] and oxometalate
superconductorsr8] are well known for nonmolecular solids;
however, there are only a few reports on other materials. The
usual method of preparation is to deposit these phases under
conditions that lead directly to the final product.
A different philosophy underlies the approach used by Johnson et al. for the controlled chemical synthesis of complex
metastable chalcogenide structures by superlattice precursors.
The concept used is based on a two-step process. In the first step
a multilayer structure is prepared by modulated sequential deposition of the chemical elements required. This structure is
subsequently transformed by low-temperature thermal conversion under defined conditions into a crystalline superlattice
compound with compositional modulation. The formation of
the heterostructure is not based on conventional epitaxial
growth but on kinetic trapping of the desired structure by the
specific solid-state reaction mechanism.
Layered transition metal dichalcogenides M X 2 of the CdI, or
MoS, structure type were selected for investigations on this
reaction model. These chalcogenides are interesting for several
reasons. Their electronic properties vary from insulator to metal
and superconductor behavior; charge density waves have been
studied extensively with these layered phases.['* '*I Their topotactic reactivity is exceptional;[", " 1 in terms of application
they are of interest, for example as electrode materials for secondary batteries and photoelectrochemical cells. as solid lubricants, and in heterogeneous catalysis. Their physical and chemical properties can be modified by isomorphous replacement of
lattice cations and anions.
VCH Verlrigsgr.srllschu/rftmhH. 0.69451 Weinheini. 1996
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Experimental investigations were initially carried out on the
hinavy Nb/Se system by X-ray diffraction and differential scanning calorimetry; these demonstrated the oriented formation of
the binary chalcogenide NbSe, as a result of the nucleation and
growth of the product along the planar interfaces of the modulated precursor phase. The major parameters for reaction control were the diffusion length (layer thickness in the precursor
state), the annealing temperature, and the annealing time. With
appropriate reaction parameters a variation of the Nb/Se ratio
(i.e. metal o r chalcogen excess relative to the 1 :2 stoichiometry)
had no influence on the formation of stoichiometric NbSe,, in
spite of the potential competition by neighboring binary phases
thermodynamically stable under these conditions. This is in perfect agreement with the assumption of a low-temperature solidstate reaction with kinetic control.
If the mechanism of the interfacial nucleation with oriented
crystal growth of kinetic products (Fig. 1) is correct, it should be
Fig. 1. The reaction pathway for the formation of chalcogenide superlattices: lowtemperature thermal transformation of amorphous chalcogenide precursor multilayers, comprising layers of niobium (black spheres) and titanium atoms (gray
spheres), each separated by intervening layers of selenium atoms (white spheres)
(left). by interfacial nucleation and oriented growth into the desired crystalline
product superlattice after annealing (right).
possible to prepare new metastable heterostructures containing
ternary compounds with modulated composition perpendicular
to the direction of the thin film basal plane. X-ray and DSC
studies on the Ti/Nb/Se system in the stoichiometric ratio used
for the formation of MX, layers demonstrated a persistence of
the precursor thin film composition modulation in the course of
the low-temperature annealing process and a continuous oriented growth of the product phase. An experiment with a precursor
superlattice sequence corresponding to 6(Ti/2 Se)- 6(Nb/2 Se)
yielded a remarkable result: the X-ray pattern of the product
phase displayed all of the calculated 62 Bragg reflections of the
desired superlattice (Fig. 2). The high quality of the reflections
allowed a Rietveld analysis that confirmed the correlation between the precursor heterostructure sequence and the superlattice ordering of the final product. As expected high-temperature
annealing of the product phase leads to quantitative conversion
into the thermodynamically stable, solid solution phase with
short periodicity and identical layer units of the composition
Tio,5Nb, $e,.
The novel concept for the controlled low-temperature solidstate synthesis of metastable phases by thin-layer precursors
modulated in the nanometer range4haracterized by the competition in time windows between short-scale diffusion and heterogeneous interface nucleation with oriented growth of a
metastable product phase-has thus been successfully applied
by the authors for the controlled preparation of novel transition
log I
Fig. 2. X-ray diffractogram of a dichalcogenide superlattice with a modulation
sequence corresponding to six NbSe, and six TiSe, layer units All 62 Bragg rcflections of the desired superlattice are shown.
metal dichalcogenide structures. A new dimension is given here
for modifying the interesting physical and topochemical properties of these materials. Transition metal dichalcogenides belong
to the category of van der Waals type layered solids; their structural morphology and bonding anisotropy naturally should favor the directed interfacial growth mechanism. A large variety
of related layered compounds with similar o r more complex
layered structures or stoichiometry are known to which this
concept could be extended (e.g. transition metal dihalides, phosphorous chalcogenides, sulfide carbides etc.). It seems most likely that this synthesis strategy can similarly be applied to the
preparation of metastable framework solids characterized by
strongly anisometric structures, such as layer-type perovskites.
The limit of application of this concept may be reached with
structurally isometric solids, in which coherent directed nucleation and growth at interfaces appears to be less probable. Further aspects of general interest related to this concept are the
investigation of dynamic interface processes (in situ diffraction
studies), nucleation phenomena, and the transformation mechanisms of amorphous phases in low-temperature solid-state reactions whose investigation with bulk material is frequently
It is important to note in this context a further line of progress
achieved recently concerning modulated materials. Present techniques for the preparation of pristine thin films of nonmolecular
solids are based essentially on physical or chemical vacuum
techniques in which atoms, molecules or cluster aggregates are
deposited from the vapor phase onto solid substrates (i.e. gas/
solid processes). A basically different approach is the use
of electrochemical techniques for thin film preparation (i.e. fluid-solid conversion). There are several advantages associated
with this technique: a) the strong tendency of electrochemically
deposited materials-including isometric systems-for epitaxial
growth, b) the possibility of working with systems not suited for
vacuum deposition, c) suppression of interdiffusion between
layers (room temperature), and d) precise control of deposition
by current density (kinetics) and electrode potential (variable
energy scale). Whereas the electrochemical deposition of multilayer systems of metals/alloys and s e m i c ~ n d u c t o r s ~has
' ~ ~been
well investigated in the last decade, recent work by Switzer and
coworkers appears to be the first successful attempt to obtain
superlattices from a complex ternary inorganic material by the
electrochemical deposition technique.[141
The major subject of these investigations was the ternary TI/
PbjO system with a fluorite type structure and a large variation
in stoichiometry. By pulsed-current o r pulsed-potential deposition from electrolytes with appropriate ionic composition, superlattices with a modulation layer unit thickness of 3 nm could
be prepared. X-ray and scanning tunneling microscopy studies
confirmed the high quality of the superlattice architecture. The
modulation wave lengths were in the order of mean free path
dimensions of electrons. The material is a degenerate n-type
semiconductor whose high electronic conductivity, which lies in
the order of that of metals, is combined with the optical properties of a semiconductor; it is of interest with respect to quantum
optical. electronic, and optoelectronic functions.
A further remarkable recent result from the work by Switzer
et al. is the pulsed anodic electrodeposition of nanometer-scale
modulated “defect chemistry superlattices” of thallium(1rr) oxide (Fig. 3).“ ’] The modulation parameters here are intrinsic
anion vacancies and cation Frenkel type defects. It could be
Fig. 3. Scheme ofdefect modulation in a thallium(~rc)oxidesuperlattice obtained by
anodic pulsed deposition. Substrate = stainless steel. A = Periodicity of layer sequence.
shown that the defect chemistry of TI,O, is a function of the
applied electrode potential : high overpotentials were found to
induce oxygen vacancy defects (T1,03 - J , whereas low overpotentials favor excess TI ions on interstitial sites of the lattice (i.e.
The examples of recent work presented here underline the
increasing involvement of chemistry in the area of nonmolecular
materials artificially modulated on the nanometer scale, and the
potential of low-temperature solid-state processes and controlled materials design. In comparison with the impressive
rapid progress in “supramolecular chemistry” it now seems that
some keys to doors are becoming available, providing access to
a comparable tailoring approach in the domain of nonmolecular solids.
German version’ Angew. Cheni 1996, I()#, 933-935
Keywords: electrochemistry materials design
thin films
superlattices .
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