The field of self-assembled monolayers (SAMs) has witnessed tremendous growth in synthetic sophistication and depth of characterization over the past 15 years.1 However, it is interesting to comment on the modest beginning and on important milestones. The field really began much earlier than is now recognized. In 1946 Zisman published the preparation of a monomolecular layer by adsorption (self-assembly) of a surfactant onto a clean metal surface.2 At that time, the potential of self-assembly was not recognized, and this publication initiated only a limited level of interest. Early work initiated in Kuhn’s laboratory at Gottingen, applying many years of experience in using chlorosilane derivative to hydrophobize glass, was followed by the more recent discovery, when Nuzzo and Allara showed that SAMs of alkanethiolates on gold can be prepared by adsorption of di-n-alkyl disulfides from dilute solutions.3 Getting away from the moisture-sensitive alkyl trichlorosilanes, as well as working with crystalline gold surfaces, were two important reasons for the success of these SAMs. Many self-assembly systems have since been investigated, but monolayers of alkanethiolates on gold are probably the most studied SAMs to date. The formation of monolayers by self-assembly of surfactant molecules at surfaces is one example of the general phenomena of self-assembly. In nature, self-assembly results in supermolecular hierarchical organizations of interlocking components that provides very complex systems.4 SAMs offer unique opportunities to increase fundamental understanding of self-organization, structure-property relationships, and interfacial phenomena. The ability to tailor both head and tail groups of the constituent molecules makes SAMs excellent systems for a more fundamental understanding of phenomena affected by competing intermolecular, molecular-substrates and molecule-solvent interactions like ordering and growth, wetting, adhesion, lubrication, and corrosion. That SAMs are well-defined and accessible makes them good model systems for studies of physical chemistry and statistical physics in two dimensions, and the crossover to three dimensions. SAMs provide the needed design flexibility, both at the individual molecular and at the material levels, and offer a vehicle for investigation of specific interactions at interfaces, and of the effect of increasing molecular complexity on the structure and stability of two-dimensional assemblies. These studies may eventually produce the design capabilities needed for assemblies of three-dimensional structures.5 However, this will require studies of more complex systems and the combination of what has been learned from SAMs with macromolecular science. The exponential growth in SAM research is a demonstration of the changes chemistry as a disciAbraham Ulman was born in Haifa, Israel, in 1946. He studied chemistry in the Bar-Ilan University in Ramat-Gan, Israel, and received his B.Sc. in 1969. He received his M.Sc. in phosphorus chemistry from Bar-Ilan University in 1971. After a brief period in industry, he moved to the Weizmann Institute in Rehovot, Israel, and received his Ph.D. in 1978 for work on heterosubstituted porphyrins. He then spent two years at Northwestern University in Evanston, IL, where his main interest was onedimensional organic conductors. In 1985 he joined the Corporate Research Laboratories of Eastman Kodak Company, in Rochester, NY, where his research interests were molecular design of materials for nonlinear optics and self-assembled monolayers. In 1994 he moved to Polytechnic University where he is the Alstadt-Lord-Mark Professor of Chemistry. His interests encompass self-assembled monolayers, surface engineering, polymers at interface, and surfaces phenomena. 1533 Chem. Rev. 1996, 96, 1533−1554

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Formation and Structure of Self-Assembled Monolayers.

The magnetoelectric effect--the induction of magnetization by means of an electric field and induction of polarization by means of a magnetic field--was first presumed to exist by Pierre Curie, and subsequently attracted a great deal of interest in the 1960s and 1970s (refs 2-4). More recently, related studies on magnetic ferroelectrics have signalled a revival of interest in this phenomenon. From a technological point of view, the mutual control of electric and magnetic properties is an attractive possibility, but the number of candidate materials is limited and the effects are typically too small to be useful in applications. Here we report the discovery of ferroelectricity in a perovskite manganite, TbMnO3, where the effect of spin frustration causes sinusoidal antiferromagnetic ordering. The modulated magnetic structure is accompanied by a magnetoelastically induced lattice modulation, and with the emergence of a spontaneous polarization. In the magnetic ferroelectric TbMnO3, we found gigantic magnetoelectric and magnetocapacitance effects, which can be attributed to switching of the electric polarization induced by magnetic fields. Frustrated spin systems therefore provide a new area to search for magnetoelectric media.

Magnetic control of ferroelectric polarization

Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examined and their properties are discussed, with particular attention to their charge density wave, superconductive and topological phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties. Two-dimensional transition metal dichalcogenides (TMDCs) exhibit attractive electronic and mechanical properties. In this Review, the charge density wave, superconductive and topological phases of TMCDs are discussed, along with their synthesis and applications in devices with enhanced mobility and with the use of strain engineering to improve their properties.

2D transition metal dichalcogenides

Frustrated magnets are materials in which localized magnetic moments, or spins, interact through competing exchange interactions that cannot be simultaneously satisfied, giving rise to a large degeneracy of the system ground state. Under certain conditions, this can lead to the formation of fluid-like states of matter, so-called spin liquids, in which the constituent spins are highly correlated but still fluctuate strongly down to a temperature of absolute zero. The fluctuations of the spins in a spin liquid can be classical or quantum and show remarkable collective phenomena such as emergent gauge fields and fractional particle excitations. This exotic behaviour is now being uncovered in the laboratory, providing insight into the properties of spin liquids and challenges to the theoretical description of these materials.

Spin liquids in frustrated magnets

▪ Abstract Soft lithography, a set of techniques for microfabrication, is based on printing and molding using elastomeric stamps with the patterns of interest in bas-relief. As a technique for fabricating microstructures for biological applications, soft lithography overcomes many of the shortcomings of photolithography. In particular, soft lithography offers the ability to control the molecular structure of surfaces and to pattern the complex molecules relevant to biology, to fabricate channel structures appropriate for microfluidics, and to pattern and manipulate cells. For the relatively large feature sizes used in biology (≥50 μm), production of prototype patterns and structures is convenient, inexpensive, and rapid. Self-assembled monolayers of alkanethiolates on gold are particularly easy to pattern by soft lithography, and they provide exquisite control over surface biochemistry.

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Soft Lithography in Biology and Biochemistry

Previously we have presented evidence for stripe order of holes and spins in ${\mathrm{La}}_{1.6\ensuremath{-}x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ with $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.12$. Here we show, via neutron diffraction measurements of magnetic scattering, that similar order occurs in crystals with $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.15$ and $0.20$. Zero-field-cooled magnetization measurements show that all three compositions are also superconducting, with the superconducting transition temperature increasing as the low-temperature staggered magnetization decreases. These results directly demonstrate an intimate connection between stripe correlations and superconductivity.

https://link.aps.org/pdf/10.1103/PhysRevLett.78.338

Coexistence of, and Competition between, Superconductivity and Charge-Stripe Order in La 1 . 6 − x Nd 0.4 Sr x CuO 4

Successive phase transformations in ${\mathrm{K}}_{2}$Se${\mathrm{O}}_{4}$ at ${T}_{i}=130$ K and ${T}_{c}=93$ K were studied by the neutron-scattering technique. The superlattice reflections in the intermediate phase were found to be incommensurate with the lattice periodicity. The wave vector characterizing the reflections is ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{\ensuremath{\delta}}=\frac{(1\ensuremath{-}\ensuremath{\delta}){\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}}^{*}}{3}$ with $\ensuremath{\delta}=0.07$ at 122.5 K. The deviation \ensuremath{\delta} decreases with decreasing temperature with an apparently discontinuous jump to zero at ${T}_{c}$. Below this temperature, the crystal remains commensurate and is known to be ferroelectric. The incommensurate-commensurate transition and the simultaneous occurrence of the commensurate phase and the spontaneous polarization are discussed using a Landau-type expansion of the free energy in which a term proportional to ${Q}^{3}({\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{\ensuremath{\delta}}){P}_{z}({\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{3\ensuremath{\delta}})$ plays an essential role in driving the incommensurate-commensurate phase transformation and in inducing the spontaneous polarization. Here, $Q({\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{\ensuremath{\delta}})$ is the amplitude of the primary atomic displacements with wave vector ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{\ensuremath{\delta}}$ and ${P}_{z}({\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{3\ensuremath{\delta}})$ is the polarization wave with wave vector ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{3\ensuremath{\delta}}=3\ensuremath{\delta}(\frac{{\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}}^{*}}{3})$ and becomes the macroscopic polarization below ${T}_{c}$. Above ${T}_{i}$, a ${\ensuremath{\Sigma}}_{2}$ optic-phonon branch along ($\ensuremath{\xi},0,0$) shows a striking softening and ${\ensuremath{\omega}}_{j}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$ for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}\ensuremath{\sim}(\frac{1}{3},0,0)$ tends to zero at ${T}_{i}$. The softening results from a temperature-dependent decrease of the interlayer forces with ranges $\frac{a}{2}$ and $a$ ($a$ is one unit-cell length along the $a$ axis) in the presence of strong and persisting forces with a range $\frac{3a}{2}$. The intensities of the soft phonon were measured about different reciprocal-lattice points and were used to determine the nature of the soft-phonon mode and suggest a coupled translation of potassium ions with rotational motion of Se${\mathrm{O}}_{4}$ groups to be the origin of the lattice instability.

Structural phase transformation in K 2 Se O 4

We present a neutron diffraction study of charge and spin order within the ${\mathrm{CuO}}_{2}$ planes of ${\mathrm{La}}_{1.48}$${\mathrm{Nd}}_{0.4}$${\mathrm{Sr}}_{0.12}$${\mathrm{CuO}}_{4}$, a crystal in which superconductivity is anomalously suppressed. At low temperatures we observe elastic magnetic superlattice peaks of the type (1/2\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon},1/2,0) and charge-order peaks at (2\ifmmode\pm\else\textpm\fi{}2\ensuremath{\epsilon},0,0), where \ensuremath{\epsilon}=0.118. After cooling the crystal through the low-temperature-orthorhombic (LTO) to low-temperature-tetragonal (LTT) phase transition near 70 K, the charge-order peaks appear first at \ensuremath{\sim}60 K, with the magnetic peaks appearing below 50 K. The magnetic peaks increase in intensity by an order of magnitude below 3 K due to ordering of the Nd ions. We show that the observed diffraction features are consistent with stripe-phase order, in which the dopant-induced holes collect in domain walls that separate antiferromagnetic antiphase domains. The Q dependence of the magnetic scattering indicates that the low-temperature correlation length within the planes is substantial (\ensuremath{\sim}170 \AA{}), but only very weak correlations exist between next-nearest-neighbor planes. Correlations between nearest-neighbor layers are frustrated by pinning of the charge stripes to the lattice distortions of the LTT phase. The spin-density-wave amplitude corresponds to a Cu moment of 0.10\ifmmode\pm\else\textpm\fi{}0.03 ${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. The behavior of the electrical resistivity within the LTT phase is examined, and the significance of stripe-phase correlations for understanding the unusual transport properties of layered cuprates is discussed. \textcopyright{} 1996 The American Physical Society.

Neutron-scattering study of stripe-phase order of holes and spins in La1.48Nd0.4Sr0.12CuO4.

Neutron-scattering studies of $2H$-Nb${\mathrm{Se}}_{2}$ and $2H$-Ta${\mathrm{Se}}_{2}$ reveal the strong Kohn anomalies and incommensurate superlattice characteristic of charge-density-wave instabilities. Whereas the Nb${\mathrm{Se}}_{2}$ superlattice (${T}_{0}=33.5$ K) remains incommensurate to 5 K, Ta${\mathrm{Se}}_{2}$ (${T}_{0}=122.3$ K) locks to a commensurate $3a$ superlattice at 90 K. The temperature dependence of the superlattice wave vector and the lockin behavior are understood using a free energy involving terms third order in atomic displacement. A secondary lattice distortion required by this model is observed.

Study of Superlattice Formation in 2 H -Nb Se 2 and 2 H -Ta Se 2 by Neutron Scattering

We have used the triple-axis neutron-scattering technique to study $2H\ensuremath{-}\mathrm{Ta}{\mathrm{Se}}_{2}$ and $2H\ensuremath{-}\mathrm{Nb}{\mathrm{Se}}_{2}$ which undergo charge-density wave transitions at ${T}_{0}=122.3$ K and ${T}_{0}=33.5$ K, respectively. The transitions in both compounds appear to be second-order and involve atomic displacements of ${\ensuremath{\Sigma}}_{1}$ symmetry. At inception, the superlattices in both compounds have nearly identical incommensurate wave vectors with magnitude ${q}_{\ensuremath{\delta}}=\frac{1}{3(1\ensuremath{-}\ensuremath{\delta}){a}^{*}}$, with $\ensuremath{\delta}\ensuremath{\sim}0.02$. The Nb${\mathrm{Se}}_{2}$ superlattice remains incommensurate to 5 K but Ta${\mathrm{Se}}_{2}$ undergoes a first-order lock-in transition where $\ensuremath{\delta}\ensuremath{\rightarrow}0$ at 90 K. The temperature dependence of the superlattice wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$ in the incommensurate phase and the lock-in transition are discussed using a free energy involving third-order "umklapp" terms and a secondary order parameter. The secondary lattice distortion which is predicted in this model is observed experimentally. Most phonon branches having energies less than 10 meV with propagation vectors in the [$\ensuremath{\zeta}00$] and [$00\ensuremath{\zeta}$] directions have been measured at 300 K. Strong anomalies are found in the ${\ensuremath{\Sigma}}_{1}[\ensuremath{\zeta}00]$ phonon branches in both materials near the wave vector ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{c}=(\frac{1}{3},0,0)$ characteristic of the low-temperature superlattices. Substantial softening of this phonon is observed as the transition is approached. In addition, the spectral profile exhibits a central peak which is not measurably inelastic.

J. D. Axe

Papers

Coexistence of, and Competition between, Superconductivity and Charge-Stripe Order in La 1 . 6 − x Nd 0.4 Sr x CuO 4

Structural phase transformation in K 2 Se O 4

Neutron-scattering study of stripe-phase order of holes and spins in La1.48Nd0.4Sr0.12CuO4.

Study of Superlattice Formation in 2 H -Nb Se 2 and 2 H -Ta Se 2 by Neutron Scattering

Neutron scattering study of the charge-density wave transitions in 2 H − Ta Se 2 and 2 H − Nb Se 2