Showing papers on "Nucleation published in 2002"

Crystalline Silver Nanowires by Soft Solution Processing

Abstract: This paper describes a soft, solution-phase approach to the large-scale synthesis of uniform nanowires of bicrystalline silver whose lateral dimensions could be controlled in the range of 30−40 nm, and lengths up to ∼50 μm. The first step of this procedure involved the formation of platinum nanoparticles by reducing PtCl2 with ethylene glycol heated to ∼160 °C. Due to their close match in crystal structure and lattice constants, these platinum nanoparticles could serve as seeds for the heterogeneous nucleation and growth of silver that was produced in the solution via the reduction of AgNO3 with ethylene glycol. When surfactants such as poly(vinyl pyrrolidone) (PVP) were present in this solution, the silver could be directed to grow into uniform nanowires with aspect ratios as high as ∼1000. Measurements of transport property at room temperature indicated that these nanowires were electrically continuous with a conductivity of approximately 0.8 × 105 S/cm.

Uniform silver nanowires synthesis by reducing AgNO3 with ethylene glycol in the presence of seeds and poly(vinyl pyrrolidone)

Abstract: A solution-phase approach has been demonstrated for the large-scale synthesis of silver nanowires with diameters in the range of 30−40 nm, and lengths up to ∼50 μm. The first step of this process involved the formation of Pt (or Ag) nanoparticles by reducing PtCl2 (or AgNO3) with ethylene glycol (EG) heated to ∼160 °C. These Pt (or Ag) nanoparticles could serve as seeds for the heterogeneous nucleation and growth of silver (formed by reducing AgNO3 with EG) because of their close match in crystal structure and lattice constants. In the presence of poly(vinyl pyrrolidone) (PVP), the growth of silver could be directed into a highly anisotropic mode to form uniform nanowires with aspect ratios as high as ∼1000. UV−visible spectroscopy, SEM, TEM, XRD, and electron diffraction were used to characterize these silver nanowires, indicating the formation of a highly pure phase, as well as a uniform diameter and bicrystalline structure. Both morphology and aspect ratios of these silver nanostructures could be varie...

Nearly Monodisperse and Shape-Controlled CdSe Nanocrystals via Alternative Routes: Nucleation and Growth

Abstract: The nucleation and growth of colloidal CdSe nanocrystals with a variety of elongated shapes were explored in detail. The critical size nuclei for the system were magic sized nanoclusters, which possessed a sharp and dominated absorption peak at 349 nm. The formation of the unique magic sized nuclei in a broad monomer concentration range was not expected by the classic nucleation theory. We propose that this was a result of the extremely high chemical potential environment, that is, very high monomer concentrations in the solution, required for the growth of those elongated nanocrystals. The shape, size, and size/shape distributions of the resulting nanocrystals were all determined by two related factors, the magic sized nuclei and the concentration of the remaining monomers after the initial nucleation stage. Without any size sorting, nearly monodisperse CdSe quantum structures with different shapes were reproducibly synthesized by using the alternative cadmium precursors, cadmium-phosphonic acid complexe...

Formation of zno nanostructures by a simple way of thermal evaporation

Abstract: Mass production of ZnO nanowires, nanoribbons, and needle-like rods has been achieved by a simple method of thermal evaporation of ZnO powders mixed with graphite. Metallic catalysts, carrying gases, and vacuum conditions are not necessary. Temperature is the critical experimental parameter for the formation of different morphologies of ZnO nanostructures. Zn or Zn suboxide plays a crucial role for the nucleation of ZnO nanostructures. The as-prepared ZnO nanowires consist of single crystalline cores and thin amorphous shells. As determined by electron diffraction, the growth direction of ZnO nanowires is [001], which has no orientation relationship with the substrate. A strong room-temperature photoluminescence in ZnO nanostructures has been demonstrated.

Atomistic mechanisms governing elastic limit and incipient plasticity in crystals

Abstract: Nanometre-scale contact experiments1,2,3,4,5,6 and simulations7,8,9,10 demonstrate the potential to probe incipient plasticity—the onset of permanent deformation—in crystals. Such studies also point to the need for an understanding of the mechanisms governing defect nucleation in a broad range of fields and applications. Here we present a fundamental framework for describing incipient plasticity that combines results of atomistic and finite-element modelling, theoretical concepts of structural stability at finite strain, and experimental analysis. We quantify two key features of the nucleation and subsequent evolution of defects. A position-sensitive criterion based on elastic stability determines the location and character of homogeneously nucleated defects. We validate this stability criterion at both the atomistic and the continuum levels. We then propose a detailed interpretation of the experimentally observed sequence of displacement bursts to elucidate the role of secondary defect sources operating locally at stress levels considerably smaller than the ideal strength required for homogeneous nucleation. These findings provide a self-consistent explanation of the discontinuous elastic–plastic response in nanoindentation measurements, and a guide to fundamental studies across many disciplines that seek to quantify and predict the initiation and early stages of plasticity.

Materials science of thin films : deposition and structure

Abstract: Foreword to First Edition Preface Acknowledgments A Historical Perspective Chapter 1 A Review of Materials Science 1.1. Introduction 1.2. Structure 1.3. Defects in Solids 1.4. Bonds and Bands in Materials 1.5. Thermodynamics of Materials 1.6. Kinetics 1.7. Nucleation 1.8. An Introduction to Mechanical Behavior 1.9. Conclusion Exercises References Chapter 2 Vacuum Science and Technology 2.1. Introduction 2.2. Kinetic Theory of Gases 2.3. Gas Transport and Pumping 2.4. Vacuum Pumps 2.5. Vacuum Systems 2.6. Conclusion Exercises References Chapter 3 Thin-Film Evaporation Processes 3.1. Introduction 3.2. The Physics and Chemistry of Evaporation 3.3. Film Thickness Uniformity and Purity 3.4. Evaporation Hardware 3.5. Evaporation Processes and Applications 3.6. Conclusion Exercises References Chapter 4 Discharges, Plasmas, and Ion-Surface Interactions 4.1. Introduction 4.2. Plasmas, Discharges, and Arcs 4.3. Fundamentals of Plasma Physics 4.4. Reactions in Plasmas 4.5. Physics of Sputtering 4.6. Ion Bombardment Modification of Growing Films 4.7. Conclusion Exercises References Chapter 5 Plasma and Ion Beam Processing of Thin Films 5.1. Introduction 5.2. DC, AC, and Reactive Sputtering Processes 5.3. Magnetron Sputtering 5.4. Plasma Etching 5.5. Hybrid and Modified PVD Processes 5.6. Conclusion Exercises References Chapter 6 Chemical Vapor Deposition 6.1. Introduction 6.2. Reaction Types 6.3. Thermodynamics of CVD 6.4. Gas Transport 6.5. Film Growth Kinetics 6.6. Thermal CVD Processes 6.7. Plasma-Enhanced CVD Processes 6.8. Some CVD Materials Issues 6.9. Safety 6.10. Conclusion Exercises References Chapter 7 Substrate Surfaces and Thin-Film Nucleation 7.1. Introduction 7.2. An Atomic View of Substrate Surfaces 7.3. Thermodynamic Aspects of Nucleation 7.4. Kinetic Processes in Nucleation and Growth 7.5. Experimental Studies of Nucleation and Growth 7.6. Conclusion Exercises References Chapter 8 Epitaxy 8.1. Introduction 8.2. Manifestations of Epitaxy 8.3. Lattice Misfit and Defects in Epitaxial Films 8.4. Epitaxy of Compound Semiconductors 8.5. High-Temperature Methods for Depositing Epitaxial Semiconductor Films 8.6. Low-Temperature Methods for Depositing Epitaxial Semiconductor Films 8.7. Mechanisms and Characterization of Epitaxial Film Growth 8.8. Conclusion Exercises References Chapter 9 Film Structure 9.1. Introduction 9.2. Structural Morphology of Deposited Films and Coatings 9.3. Computational Simulations of Film Structure 9.4. Grain Growth, Texture, and Microstructure Control in Thin Films 9.5. Constrained Film Structures 9.6. Amorphous Thin Films 9.7. Conclusion Exercises References Chapter 10 Characterization of Thin Films and Surfaces 10.1. Introduction 10.2. Film Thickness 10.3. Structural Characterization of Films and Surfaces 10.4. Chemical Characterization of Surfaces and Films 10.5. Conclusion Exercises References Chapter 11 Interdiffusion, Reactions, and Transformations in Thin Films 11.1. Introduction 11.2. Fundamentals of Diffusion 11.3. Interdiffusion in Thin Metal Films 11.4. Compound Formation and Phase Transformations in Thin Films 11.5. Metal-Semiconductor Reactions 11.6. Mass Transport in Thin Films under Large Driving Forces 11.7. Conclusion Exercises References Chapter 12 Mechanical Properties of Thin Films 12.1. Introduction 12.2. Mechanical Testing and Strength of Thin Films 12.3. Analysis of Internal Stress 12.4. Techniques for Measuring Internal Stress in Films 12.5. Internal Stresses in Thin Films and Their Causes 12.6. Mechanical Relaxation Effects in Stressed Films 12.7. Adhesion 12.8. Conclusion Exercises References Index

Preparation of Layered Double-Hydroxide Nanomaterials with a Uniform Crystallite Size Using a New Method Involving Separate Nucleation and Aging Steps

Abstract: In particle formation, the method can be just as important as the chemical reaction involved. A new method of synthesizing hydrotalcite-like layered double hydroxides (LDHs) of the type [Mg1-xAlx(OH)2]x+(CO32-)x/2·yH2O (x = 1.7−3.3) is reported. The key features of this method are a very rapid mixing and nucleation process in a colloid mill followed by a separate aging process. The properties of the resulting LDHs are compared with those of materials produced using the conventional coprecipitation process at constant pH. The compositions and structural parameters of the materials synthesized using the two routes are very similar, although the crystallinity is slightly higher for the LDHs produced using the new method. The thermal behavior of the materials synthesized using the two routes is also similar. The major advantage of the new method is that it affords smaller crystallites with a higher aspect ratio, having a very narrow distribution of crystallite size. In the conventional coprecipitation process...

Growth conditions for wurtzite zinc oxide films in aqueous solutions

Abstract: A pH above 9.0 was essential for the formation of wurtzite zinc oxide (ZnO) in aqueous solution systems. The addition of complexing agents to decrease the deposition rate was required for the direct growth of ZnO on the surface of substrates through heterogeneous nucleation. The nucleation of the crystals was promoted by undercoats derived from zinc acetate. Wurtzite ZnO films consisting of hexagonal columns with diameters of 20–100 nm were successfully prepared on various substrates under suitable conditions.

An improved parameterization for sulfuric acid-water nucleation rates for tropospheric and stratospheric conditions

Abstract: [1] In this paper we present parameterized equations for calculation of sulfuric acid–water critical nucleus compositions, critical cluster radii and homogeneous nucleation rates for tropospheric and stratospheric conditions. The parameterizations are based on a classical nucleation model. We used an improved model for the hydrate formation relying on ab initio calculations of small sulfuric acid clusters and on experimental data for vapor pressures and equilibrium constants for hydrate formation. The most rigorous nucleation kinetics and the thermodynamically consistent version of the classical binary homogeneous nucleation theory were used. The parameterized nucleation rates are compared with experimental ones, and at room temperature and relative humidities above 30% they are within experimental error. At lower temperatures and lower humidities the agreement is somewhat poorer. Overall, the values of nucleation rates are increased compared to a previous parameterization and are within an order of magnitude compared with theoretical values for all conditions studied. The parameterized equations will reduce the computing time by a factor 1/500 compared to nonparameterized nucleation rate calculations and therefore are in particular useful for large-scale models. The parameterized formulas are valid at temperatures between 230.15 K and 305.15 K, relative humidities between 0.01% and 100%, and sulfuric acid concentrations from 104 to 1011 cm−3. They can be used to extrapolate the classical results down to 190 K. The parametrization is limited to cases where nucleation rates are between 10−7 and 1010 cm−3s−1, and the critical cluster contains at least four molecules.

Direct Assembly of Large Arrays of Oriented Conducting Polymer Nanowires

Abstract: Although oriented carbon nanotubes, oriented nanowires of metals, semiconductors and oxides have attracted wide attention, there have been few reports on oriented polymer nanostructures such as nanowires. In this paper we report the assembly of large arrays of oriented nanowires through controlled nucleation and growth during a stepwise electrochemical deposition process in which a large number of nuclei were first deposited on the substrate using a large current density. After the initial nucleation, the current density was reduced step by step to grow the oriented nanowires from the nucleation sites created in the first step. A very different morphology was also demonstrated by first depositing a monolayer of close-packed colloidal spheres using a similar step-wise deposition process. As a result, the polymer nanofibers grew from the spheres in a radial fashion and formed the continuous three-dimensional network of nanofibers in the film. The principles of control nucleation and growth in electrochemical deposition investigated in this paper should be applicable to other electrical conducting and electrochemical active materials, including metals and conducting oxides. We also hope the oriented electroactive polymer nanostructure will open the door for new applications, such as miniaturized biosensors.

Non-Kolmogorov-Avrami switching kinetics in ferroelectric thin films

Abstract: The switching kinetics in ferroelectric thin films has been intensively studied during the past decade. It is widely accepted that this kinetics is basically governed by the dynamics of domain coalescence (the Kolmogorov-Avrami-Ishibashi model). This conclusion is mainly supported by fitting the time dependence of the switching currents to that predicted by this model, the fit being typically performed in a 1-2 decade interval of time. The present paper reports on a study of the switching kinetics in modified Pt/Pb(Zr,Ti)O-3/Pt thin films as a function of time and applied voltage, performed in time intervals from 10 ns to 1s. Our experimental data show that both the time and applied field dependences of the switching polarization (when monitored over a wide enough time interval) are in a strong qualitative disagreement with the predictions of the Kolmogorov-Avrami-Ishibashi approach. For the interpretation of our result, an alternative approach is forwarded. In contrast to Kolmogorov-Avrami-Ishibashi approach, we assume that the film consists of many areas, which have independent switching dynamics. The switching in an area is considered to be triggered by an act of the reverse domain nucleation. The switching kinetics is described in terms of the distribution function of the nucleation probabilities in these areas. The developed approach enables a good description of the polarization dynamics in typical ferroelectric thin films for memory applications.

Mesoscopic Metal Particles and Wires by Electrodeposition

Abstract: The electrodeposition of metal onto a low energy electrode surface like graphite or H-terminated silicon produces mesoscopic metal particles that are broadly distributed in diameter Broad size distributions are observed even in cases where the nucleation of metal is temporally controlled For this reason, electrodeposition has been infrequently used as a means for obtaining metal nanostuctures The central problem is the diffusional “cross-talk” that exists between neighboring metal nanostructures on the electrode surface Evidence for this diffusional interparticle coupling is encoded into particle size and position distributions obtained from experimental data and from Brownian Dynamics computer simulations of nanostructure growth Diffusional cross-talk between nanostructures can be turned off using either of two growth strategies described in this paper These methods permit the size-selective electrodeposition of metal nanoparticles and nanowires that are narrowly distributed in diameter

An experimental study of the kinetics of decompression-induced crystallization in silicic melt

Abstract: [1] Experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression. Pinatubo dacite was held at 780°C, 220 MPa, fO2 = NNO + 2, H2O-saturated conditions for an equilibration period, decompressed to final pressures, Pf, ranging from 175 to 5 MPa, and then held for 0.3–931 hours. According to the plagioclase liquidus curve in PH2O-T space for the relevant melt composition, these decompressions impose effective undercoolings, ΔTeff, of 34–266°C. Growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 ≤ Pf < 150 MPa (ΔTeff = 34–93°C), and equilibrium crystal modes are achieved in <168 hours. Microlite nucleation is the dominant transformation process for 10 < Pf < 50 MPa (ΔTeff = 125–241°C), and chemical equilibrium is not attained by 168 hours under these conditions. Slow, steady decompressions typically produced normally zoned, euhedral, and planar-faceted feldspar crystals, although anhedral morphologies were produced at very low Pf. Contrary to expectation, slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure. Although counterintuitive, these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth (controlled by ΔTeff and melt viscosity) experienced during each decompression path. Analysis of liquid to solid transformation kinetics using TTT-style diagrams shows that crystallization occurs most rapidly at ∼100 MPa by a crystal growth mechanism. The next most efficient crystallization conditions are at 25 MPa, in a crystal nucleation-dominated regime.

Site-specific growth of Zno nanorods using catalysis-driven molecular-beam epitaxy

Abstract: We report on catalyst-driven molecular beam epitaxy of ZnO nanorods. The process is site specific, as single crystal ZnO nanorod growth is realized via nucleation on Ag films or islands that are deposited on a SiO2-terminated Si substrate surface. Growth occurs at substrate temperatures on the order of 300–500 °C. The nanorods are uniform cylinders, exhibiting diameters of 15–40 nm and lengths in excess of 1 μm. With this approach, nanorod placement can be predefined via location of metal catalyst islands or particles. This, coupled with the relatively low growth temperatures needed, suggests that ZnO nanorods could be integrated on device platforms for numerous applications, including chemical sensors and nanoelectronics.

Grain nucleation and growth during phase transformations.

Abstract: The mechanical properties of polycrystalline materials are largely determined by the kinetics of the phase transformations during the production process. Progress in x-ray diffraction instrumentation at synchrotron sources has created an opportunity to study the transformation kinetics at the level of individual grains. Our measurements show that the activation energy for grain nucleation is at least two orders of magnitude smaller than that predicted by thermodynamic models. The observed growth curves of the newly formed grains confirm the parabolic growth model but also show three fundamentally different types of growth. Insight into the grain nucleation and growth mechanisms during phase transformations contributes to the development of materials with optimal mechanical properties.

Size effects in the structural phase transition of VO 2 nanoparticles

Abstract: We have observed size effects in the structural phase transition of submicron vanadium dioxide precipitates in silica. The ${\mathrm{VO}}_{2}$ nanoprecipitates are produced by the stoichiometric coimplantation of vanadium and oxygen and subsequent thermal processing. The observed size dependence in the transition temperature and hysteresis loops of the semiconductor-to-metal phase transition in ${\mathrm{VO}}_{2}$ is described in terms of heterogeneous nucleation statistics with a phenomenological approach in which the density of nucleating defects is a power function of the driving force.

Islet amyloid: phase partitioning and secondary nucleation are central to the mechanism of fibrillogenesis.

Abstract: Islet amyloid polypeptide (IAPP) contributes to the pathogenesis of type II diabetes by depositing as cytotoxic amyloid fibers in the endocrine pancreas. Fiber formation occurs with a marked conformational change from an unstructured precursor. Using real-time quantitative kinetic methods, fibrillogenesis was characterized as a function of protein, denaturant, and seed concentration. Several observations are in sharp contrast to the expectations for nucleation-dependent polymerization. First, the half-time of conversion for both de novo and seeded kinetics were found to be independent of protein concentration. Second, while elongation kinetics scale linearly with protein concentration, they are relatively insensitive to changes in the total seed concentration. Third, seeded bypass of de novo fiber formation kinetics shows a lag phase. The seeded lag phase is eliminated by a time delay before the introduction of seed to a de novo reaction. Last, conversion is highly cooperative, with the time required for 10-90% conversion occurring much faster than the lag time. At a minimum, four kinetic steps are required to describe these observations: activation, fiber independent nucleation, fiber-dependent nucleation, and elongation. Furthermore, we invoke a phase transition in which protein initially forms an off-pathway dispersion. This single construct allows us to model both the concentration independence of the de novo reaction time and the first-order concentration dependence of the elongation kinetics. Marked acceleration of this reaction by hexafluoro-2-propanol reinforces this view by altering the relative solubility of the two phases and/or by stabilizing hydrogen-bonded structures in the transition states of the reaction pathway.

DNA as a Selective Metallization Template

Abstract: The microscopic mechanism of platinum cluster nucleation on DNA templates is studied by first-principle molecular dynamics simulations. We find that Pt(II) complexes bound to DNA can form strong Pt−Pt bonds with free Pt complexes after a single reduction step, and may thus act as preferential nucleation sites. This is confirmed by a series of reduction experiments, in which we achieve purely heterogeneous platinum growth on DNA, and use it to fabricate metal cluster necklaces of unprecedented thinness and regularity.

Onset of Nucleate Boiling and Active Nucleation Site Density During Subcooled Flow Boiling

Abstract: The partitioning of the heat flux supplied at the wall is one of the key issues that needs to be resolved if one is to model subcooled flow boiling accurately. The first step in studying wall heat flux partitioning is to account for the various heat transfer mechanisms involved and to know the location at which the onset of nucleate boiling (ONB) occurs. Active nucleation site density data is required to account for the energy carried away by the bubbles departing from the wall. Subcooled flow boiling experiments were conducted using a flat plate copper surface and a nine-rod (zircalloy-4) bundle. The location of ONB during the experiments was determined from visual observations as well as from the thermocouple output. From the data obtained it is found that the heat flux and wall superheat required for inception are dependent on flow rate, liquid subcooling, and contact angle. The existing correlations for ONB underpredict the wall superheat at ONB in most cases. A correlation for predicting the wall superheat and wall heat flux at ONB has been developed from the data obtained in this study and that reported in the literature. Experimental data are within630 percent of that predicted from the correlation. Active nucleation site density was determined by manually counting the individual sites in pictures obtained using a CCD camera. Correlations for nucleation site density, which are independent of flow rate and liquid subcooling, but dependent on contact angle have been developed for two ranges of wall superheat—one below 15°C and another above 15°C. @DOI: 10.1115/1.1471522#

Nucleation and growth of germanium nanowires seeded by organic monolayer-coated gold nanocrystals.

Abstract: Germanium nanowires, ranging from 10 to 150 nm in diameter, were grown several micrometers in length in cyclohexane heated and pressurized above its critical point. Alkanethiol-protected gold nanocrystals, either 2.5 or 6.5 nm in diameter, were used to seed wire formation. Growth proceeded through a solution−liquid−solid mechanism at growth temperatures ranging from 300 to 450 °C. At temperatures exceeding 500 °C, large Ge particulates formed due to unfavorable growth kinetics. Temperature, the nature of the precursor, precursor concentration, and the Au:Ge ratio were determining factors in nanowire morphology. The Ge nanowires were characterized using a range of techniques, including XPS, XRD, high-resolution TEM and SEM, nanometer-scale EDS mapping, and DTA.

Nucleation and growth of atomic layer deposited HfO2 gate dielectric layers on chemical oxide (Si–O–H) and thermal oxide (SiO2 or Si–O–N) underlayers

Abstract: A study was undertaken to determine the efficacy of various underlayers for the nucleation and growth of atomic layer deposited HfO2 films. These were compared to films grown on hydrogen terminated Si. The use of a chemical oxide underlayer results in almost no barrier to film nucleation, enables linear and predictable growth at constant film density, and the most two-dimensionally continuous HfO2 films. The ease of nucleation is due to the large concentration of OH groups in the hydrous, chemical oxide. HfO2 grows on chemical oxide at a coverage rate of about 14% of a monolayer per cycle, and films are about 90% of the theoretical density of crystalline HfO2. Growth on hydrogen terminated Si is characterized by a large barrier to nucleation and growth, resulting in three-dimensional, rough, and nonlinear growth. Thermal oxide/oxynitride underlayers result in a small nucleation barrier, and nonlinear growth at low HfO2 coverages. The use of chemical oxide underlayers clearly results in the best HfO2 layers. Further, the potential to minimize the chemical oxide thickness provides an important research opportunity for high-κ gate dielectric scaling below 1.0 nm effective oxide thickness.

A new approach for studying nucleation phenomena using molecular simulations: Application to CO2 hydrate clathrates

Abstract: We use an order-parameter formulation, in conjunction with non-Boltzmann sampling to study the nucleation of clathrate hydrates from water–CO2 mixtures, using computer simulations. A set of order parameters are defined: Φigg (i=1,2,…,n and gg for guest–guest), which characterize the spatial and orientational order of the CO2 molecules, and Φihh (hh for host–host), which govern the ordering of the water molecules. These are bond-orientational order parameters based on the average geometrical distribution of nearest-neighbor bonds. The free-energy hypersurface as a function of the order parameters is calculated using the Landau–Ginzburg approach. The critical cluster size that leads to the nucleation of the clathrate phase is determined accurately by analyzing the free energy surface. We find that the nucleation proceeds via “the local structuring mechanism,” i.e., a thermal fluctuation causing the local ordering of CO2 molecules leads to the nucleation of the clathrate, and not by the current conceptual pi...

Kinetics of capillary condensation in nanoscopic sliding friction.

Abstract: The velocity and humidity dependence of nanoscopic sliding friction has been studied on CrN and diamondlike carbon surfaces with an atomic force microscope. The surface wettability is found to be decisive. Partially hydrophilic surfaces show a logarithmic decrease of friction with increasing velocity, the slope of which varies drastically with humidity, whereas on partially hydrophobic surfaces we confirm the formerly reported logarithmic increase. A model for the thermally activated nucleation of water bridges between tip and sample asperities fully reproduces the experimental data.