Heterogeneous ice nucleation is at the heart of a wide range of technological and natural fields ranging from cryopreservation 1 and the development of anti-icing coatings 2–4 to the glaciation of clouds 5–11 . The latter process has a profound impact on Earth’s climate as it changes major physical properties, such as the albedo and precipitation efficiency 6,7 . Of the many different ice-nucleating mineral particles, dust from feldspar mineral microcline stands out as being particularly active 11–17 . However, why microcline outperforms other particles remains a puzzle. Here, we use nanoscale imaging to show that the (001) surface of microcline is able to induce ice nucleation in the absence of active sites. In contrast, for the closely related feldspar sanidine, ice nucleation is dominant at step edges as expected. Atomistic simulations suggest that the ice nucleation is induced by a lattice match between the higher index (10.4) plane of hexagonal ice and microcline. Our findings provide a nanoscopic explanation for microcline’s exceptional ice nucleation ability and demonstrate the importance of taking higher-index surfaces into account. We expect that the latter insight will enhance our understanding of nucleation processes in nature and our ability to create materials with outstanding ice nucleating abilities.

Ice nucleation is ubiquitous in nature and technology and decisive for a wide range of fields, including biology, geochemistry and environmental science. In most cases, ice nucleation occurs heterogeneously due to the presence of an ice nucleating material. Despite its importance, we still largely fail to reliably predict the ice nucleation efficacy of a given material. A particularly puzzling example is calcite, a major constituent of rocks in the Earth's crust. Although calcite possesses a high hydrophilicity, it is basically inactive in ice nucleation. Here, we combine non-contact atomic force microscopy with temperature programmed desorption measurements to investigate water multilayer formation and ice growth on calcite's (10.4) surface in ultrahigh vacuum. Adsorption of water results in the formation of up to four water layers. Interestingly, the fourth layer is observed to be metastable, as it shrinks as soon as crystalline ice nucleates. The molecular structure of the water layers is governed by the underlying calcite lattice, highlighting the strong impact of the surface. This strong templating effect of the substrate prevents the water from adopting an ice-like structure even in the fourth layer. Our results, thus, provide an explanation for the poor ice-nucleating efficacy of calcite and underscore that ice nucleation efficacy cannot be predicted based on a single descriptor.

Silver iodide (AgI) has long been known as a material that induces precipitation in clouds. The superior ice nucleation ability is commonly explained by the close lattice match between β-AgI(0001) and the basal plane of ice. However, this surface is polar, indicating that a stabilization mechanism should be present. Here, we investigate the Ag-terminated β-AgI(0001) and the I-terminated β-AgI(000-1) surfaces in water and compare the results with images recorded in n-dodecane, 0.1 M NaCl, and 0.1 M KI aqueous solutions. Strikingly, high-resolution images consistently reveal a bulk-truncated structure. These findings and observations at a larger scale indicate the absence of any periodic or triangular surface reconstruction. Although the observed structures reveal a variety of features and depend on the surface termination, some general conclusions can be drawn. First, in solvents with poor AgI solubility, only minor changes are observed. Second, the images taken in KI solution exhibit patterns that are characteristic of mineral dissolution and growth. Thus, even in a situation where AgI can and does dissolve, no indication for a surface reconstruction can be found. These results shed new light onto the surface structure of β-AgI(0001), challenging the expectation that a surface reconstruction is present under ambient conditions.

Solid-liquid interfaces are pivotal in a broad range of fields, including electrochemistry, catalysis and geochemistry. In electrochemistry, the molecular structure of the electrode-water interface is decisive for understanding and improving electrochemical reactions. In this context, the Au(111) surface constitutes a prototypical model system as gold is believed to be a relatively inert and well-characterized sample material. So far, however, little is known about the hydration structure at the Au(111)-water interface at the molecular level. Here, we present atomic force microscopy data on a gold sample carefully cleaned according to standard electrochemistry preparation recipes. Our three-dimensional atomic force microscopy data reveal a vertically ordered hydration structure with layer-to-layer distances of 0.36 nm as expected for water. Strikingly, despite our extensive cleaning efforts, our two-dimensional data (images) reveal stripe-like structures at the Au(111)-water interface. The dimensions of these structures differ from the well-known herringbone reconstruction of the gold surface, which is why we interpret them as surface contamination. Our work, hence, demonstrates that the presence of a hydration structure is no evidence for a clean gold surface.

Abstract Of the many different ice-nucleating mineral particles, dust from feldspar mineral microcline stands out as being particularly active. However, why microcline outperforms other particles remains a puzzle. Here, we use nanoscale imaging to show that the (001) surface of microcline can induce ice nucleation even in the absence of active sites. In contrast, for the closely related feldspar sanidine, ice nucleation is dominant at step edges as expected. Atomistic simulations suggest that the ice nucleation is induced by a lattice match between the higher-index (10.4) plane of hexagonal ice and microcline. Our findings provide a nanoscopic explanation for microcline’s exceptional ice nucleation ability and demonstrate the importance of taking higher-index surfaces into account.

Feldspar minerals constitute an abundant group of tectosilicates in the Earth's crust. Consequently, feldspars play a significant role in a plethora of geochemical processes, including weathering, which results in carbon dioxide removal from the atmosphere by the formation of carbonates. Moreover, feldspar dusts are known as highly efficient ice nucleating particles, having a significant impact on the physical properties of mixed-phase clouds. For these processes, the interaction of water with the feldspar surface is decisive. However, little is known about the interaction of water with feldspar surfaces. More specifically, experimental data addressing the binding and in particular the desorption of the first water layer are sparse. Here, we present temperature-programmed desorption (TPD) experiments of water desorbing from the thermodynamically most stable cleavage plane of potassium-rich feldspar, microcline (001). From the interplay of these experimental data with density-functional theory (DFT) results we shed light onto the binding of the first water layer on microcline (001). The coverage-dependent TPD spectra reveal a gradual shift of the peak position from initially 235 K for low coverages towards lower temperatures until a coverage of four water molecules per primitive unit cell is reached. Above this coverage, the peak position remains fixed at about 180 K, even for high coverages. These results are in perfect agreement with DFT simulations, revealing a decrease in the adsorption energy with increasing coverage. When four water molecules per primitive unit cell are reached, the first layer is saturated and further water starts occupying the second layer. Our work confirms previous theory results from the literature and provides molecular-scale insights into the binding of water onto microcline (001).

Ice and mixed-phase clouds can form at moderate supercooling on seed particles through heterogeneous ice nucleation, but despite numerous experimental and computational investigations, understanding heterogeneous ice nucleation remains one of the great challenges in atmospheric science. While feldspar mineral dust particles have been identified as particularly good ice nucleating particles, they can exhibit different chemical composition and crystal structure, making it difficult to determine the atomistic details of the ice nucleation mechanism, both experimentally, and computationally. Here, we present systematic atomistic molecular dynamics studies of hydration layer structures at the interfaces of K-feldspar maximum microcline (001), (010), and (100) surfaces and water, at room temperature and moderate supercooling. Simulations on the fully hydroxylated α-terminated (001) cleavage plane reveal a complex lateral structure in the first water layer and a less ordered second layer. At room temperature, water exchange within the first hydration layer and between the first and second hydration layers occurs on a sub-nanosecond timescale. We also observe that surface potassium ions can go into solution and return to vacant surface sites on a timescale of tens of nanoseconds, but this causes surprisingly minor perturbations within the first hydration layer if the sampling time is sufficient. Hydration layer structures from simulation are in very good agreement with 3D atomic force microscopy data recently obtained for the first time on a freshly cleaved microcline surface in pure water (Dickbreder et al., 2024) – validating the accuracy of the atomistic model and providing an interpretation of the experimental data. However, the simulated hydration layer structures on the low energy (001) or (010) surfaces do not exhibit a lattice match with faces of cubic or hexagonal ice. Only the higher energy (100) surface with slightly strained lattice parameters can stabilize an ice interface at moderate supercooling in the simulations. Our results confirm previous findings (Kiselev et al., 2017; Soni and Patey, 2019) and indicate that the good ice nucleating properties of feldspars likely result from more complex active sites, possibly involving changes in surface chemistry, or topographic features such as defects, strained lattices, or step edges, which we are currently investigating. Dickbreder, T., Sabath, F., Reischl, B., Nilsson, R. V. E., Foster, A., Bechstein, R. and Kühnle, A.: Atomic structure and water arrangement on K-feldspar microcline (001), accepted in Nanoscale, DOI:10.1039/d3nr05585j, 2024. Kiselev, A., Bachmann, F., Pedevilla, P., Cox, S. J., Michaelides, A., Gerthsen, D., and Leisner, T.: Active sites in heterogeneous ice nucleation—the example of K-rich feldspars, Science, 355, 367–371, 2017. Soni, A. and Patey, G. N.: Simulations of water structure and the possibility of ice nucleation on selected crystal planes of K-feldspar, J. Chem. Phys., 150, 214501, 2019.

The aggregate state of water in clouds has a fundamental impact on the clouds’ properties such as reflectivity and lifetime. Consequently, it is crucial for the development and improvement of climate models to understand the mechanism of ice nucleation under atmospheric conditions. Most atmospheric ice nucleation is heterogeneous caused by the interaction between water droplets and ice nucleating particles. Under mixed-phase cloud conditions, one of the most important ice nucleating particles are feldspar minerals. Recent scanning electron microscopy studies have shown that ice nucleation on cleavage planes of K-rich feldspars predominantly takes place at step edges and pores (Kiselev, 2017). This has also been confirmed by video and atomic force microscopy on the micrometer scale (Holden, 2019). However, experimental insights into the atomic-scale structure of the most ice-nucleation active K-feldspar microcline are still missing, and, thus, the mechanism behind ice nucleation on feldspar minerals remains elusive. Here, we present high-resolution atomic force microscopy (AFM) data revealing the atomic structure of the microcline (001) surface in its pristine state and in contact with water (Dickbreder, 2024). AFM images of the pristine microcline (001) surface kept under ultrahigh-vacuum conditions, reveal features consistent with a hydroxyl-terminated surface. This finding suggests that water in the residual gas readily reacts with the surface highlighting the high reactivity of the as-cleaved surface. Indeed, corresponding density functional theory calculations confirm a dissociative water adsorption. Three-dimensional AFM measurements performed at the mineral-water interface unravel a layered hydration structure with two features per surface unit cell. Comparison with MD calculations suggest that the structure observed in AFM corresponds to the second hydration layer rather than the first water layer. We are convinced that the combination of structural information of the pristine and water-covered microcline (001) surface will contribute to uncovering the atomic-scale mechanism behind the exceptional ice-nucleation activity of feldspar minerals. References:Atkinson, J. D., Murray, B. J., Woodhouse, M. T., Whale, T. F., Baustian, K. J., Carslaw, K. S., Dobbie, S., O’Sullivan, D., Malkin, T. L., Nature, 498, 355-358, 2013.Dickbreder, T., Sabath, F., Reischl, B., Nilsson, R. V. E., Foster, A., Bechstein, R. and Kühnle, A., Nanoscale, DOI:10.1039/d3nr05585j, 2024.Holden, M. A., Whale, T. F., Tarn, M. D., O’Sullivan, D., Walshaw, R. D., Murray, B. J., Meldrum, F. C., Christenson, H. K., Science Advances, 5, 4316, 2019.Kiselev, A., Bachmann, F., Pedevilla, P., Cox, S. J., Michaelides, A., Gerthsen, D., and Leisner, T., Science, 355, 367–371, 2017.

Calcite (calcium carbonate) is the most abundant carbonate in the Earth's crust. Due to its omnipresence it plays a prominent role in fields such as geochemistry, biomineralization and industrial processes. Moreover, the interaction of water with the most stable cleavage plane, calcite (10.4), has been studied intensively, elucidating atomic-scale details of water binding and structure formation on this surface. Interestingly, calcite (10.4) reconstructs under ultrahigh vacuum conditions, exhibiting a (2 x 1) surface unit cell. Although first indications of this reconstruction have been presented more than 20 years ago, a clear confirmation of the existence has been provided only very recently. Here, we study the tip-assisted diffusion of water molecules on calcite (10.4) under ultrahigh vacuum conditions. By recording images series using dynamic atomic force microscopy we follow the movement of water molecules on the surface kept at 140 K. Analyzing the change in consecutive images allows for elucidating details of the molecular movement on the surface. Most notably, the analysis reveals that water molecules occupy one type of adsorption position exclusively, while the other type is not adopted. Our analysis thus demonstrates that the (2 x 1) reconstruction manifests itself in the movement of single water molecules on this surface.

Feldspar minerals are abundant rock-forming minerals playing a central role in environmental processes such as silicate weathering and ice nucleation in mixed-phase clouds. These processes typically take place at the feldspar-water interface, which is why the micro- and nanoscopic surface topology is of major importance to understand them. However, especially in the field of ice nucleation on feldspar surfaces, most experimental studies are limited to the micrometre scale, while the ice nucleation sites are expected to be nanometre sized. Here, we report an intrinsic island structure observed on microcline (001). High-resolution atomic force microscopy (AFM) images taken in ultra-high vacuum show nanometre-sized islands on the terraces and at the step edges. Atomic scale images revealing a very similar contrast and identical lattice parameters on the terrace and on the islands suggest that these islands are an intrinsic feature of the microcline surface. Moreover, AFM at the solid-liquid interface demonstrates that the observed nanostructure is stable in water. As the existence of islands significantly increases the density of edge and kink sites, the observed nanostructure might have important implications for surface reactivity and potentially ice nucleation efficiency.

Investigating the water structure at the K-feldspar microcline (001) surface is a prerequisite for understanding fundamental surface processes.

The adsorption of water on calcite(104) is investigated in ultra-high vacuum by density functional theory (DFT) and non-contact atomic force microscopy (NC-AFM) in the coverage regime of up to one monolayer (ML). DFT calculations reveal a clear preference for water to adsorb on the bulk-like carbonate group rows of the (2 × 1) reconstructed surface. Additionally, an apparent water attraction due to carbonate group reorientation suggest island formation for water adsorbed on the reconstructed carbonate group rows. Experimentally, water is found to exclusively occupy specific positions within the (2 × 1) unit cell up to 0.5 ML, to form islands at coverage between 0.5 and 1 ML, and to express a (1 × 1) structure at coverage of a full monolayer.

Desorption of molecules from surfaces is widespread both in nature and technology. Despite its omnipresence and conceptual simplicity, fundamental details can be surprisingly complex and are often poorly understood. In many cases, first-order kinetics is assumed, which implies that the adsorbates do not interact with each other and desorption is the rate-limiting process. While this might be a good approximation in some cases, it is far from reality in the case of adsorbates that form ordered structures. Here, we study the desorption of a submonolayer film of 3-nitrophenol from the natural cleavage plane of calcite kept in ultrahigh vacuum. Interestingly, two distinctly different desorption regimes are observed during isothermal desorption monitored by dynamic atomic force microscopy. Initially, at high coverages, the coverage decreases almost linearly in time, indicating a constant desorption rate. Beyond this linear regime, at low coverages, a drastic increase in desorption rate is observed until the surface is completely empty. The transition between these two regimes is associated with a critical island width. We propose an existence of a long-range attractive interaction between the molecules as a possible explanation for the sudden increase in the desorption rate when a critical island width is reached. The herein observed phenomenon of two different desorption regimes is expected to be of general nature when interactions beyond next-neighbour attraction are present. Isothermal desorption of molecules from a surface exhibits two distinctly different desorption regimes. A long-range attraction is a plausible explanation for the existence of these two regimes.

Correction for 'Water adsorption lifts the (2 × 1) reconstruction of calcite(104)' by Jonas Heggemann et al., Phys. Chem. Chem. Phys., 2024, 26, 21365-21369, https://doi.org/10.1039/D3CP01408H.

Molecular self-assembly is considered a promising tool for creating functional molecular structures on surfaces, e.g., for future molecular electronic devices and sensor applications. In molecular self-assembly, the target structure is encoded in the molecular building blocks, which are driven toward their thermodynamically favored arrangement on the surface. This approach is, however, intrinsically limited to a single molecular pattern on the surface. Structures beyond this thermodynamic ground state minimizing the free energy might become accessible through competing pathways. Here, we make use of different sample preparation pathways for arriving at distinctly different molecular structures of C-60 on the (111) cleavage plane of the calcium fluoride. Using dynamic atomic force microscopy operated in an ultrahigh vacuum, we investigate the resulting island geometries as a function of the preparation pathway. When deposited onto the surface at low temperatures (about 120 K), C-60 forms single-layer hexagonal islands. Upon heating to about 320 K, these islands transform into double-layer islands with irregular edges (two-step experiment). Interestingly, distinctly different, truncated triangular double-layer islands are obtained from a direct preparation pathway (one-step experiment), i.e., when the molecules are deposited onto the sample kept at 320 K. This pathway dependence demonstrates that nonequilibrium structures are involved. Our results are corroborated by kinetic Monte Carlo simulations, which reveal the same pathway-dependent structures as those in the experiments. Based on the simulations, we identify the barrier for freely diffusing molecules to jump from the first into the second layer (ascension barrier) as key for the formation of the different island morphologies. The second ingredient to arrive at different islands is that edge diffusion rates for single-layer and double-layer islands differ. While edge diffusion is enabled for single-layer islands, it is greatly suppressed in the case of double-layer islands (essentially due to additional bonds formed with molecules in the second layer). Thus, a given island shape is effectively stabilized when molecules can jump into the second layer. In essence, allowing the molecules to ascend into the second layer provides a means to stabilize the prepared molecular islands. Our work illustrates how different preparation protocols can be used to enhance the structural variability of molecular structure formation on surfaces.

Calcite is the most abundant carbonate mineral in Earth's crust. Upon cleavage, the (10.4) plane with a rectangular unit cell is exposed. Interestingly, several experiments suggest a (2 × 1) surface reconstruction. However, clear experimental evidence and a theoretical confirmation were long missing. Recently, convincing indication for a (2 × 1) reconstruction has been given by atomic force microscopies taken at 5 K. Here, we show temperature-programmed desorption (TPD) experiments of water and ethanol desorbing from calcite (10.4) around room temperature. The TPD curves fit excellently to a kinetic model considering two different adsorption sites, as expected in case of a (2 × 1) reconstruction. This finding applies to the desorption of water and ethanol, illustrating that the effect is characteristic for the calcite cleavage plane. Our results thus show that the (2 × 1) reconstruction not only exists at room temperature but has significant impact on the interfacial properties of calcite.

Scanning probe microscopy (SPM) techniques are widely used to study the structure and properties of surfaces and interfaces across a variety of disciplines in chemistry and physics. One of the major artifacts in SPM is (thermal) drift, an unintended movement between sample and probe, which causes a distortion of the recorded SPM data. Literature holds a multitude of strategies to compensate for drift during the measurement (online drift correction) or afterwards (offline drift correction). With the currently available software tools, however, offline drift correction of SPM data is often a tedious and time-consuming task. This is particularly disadvantageous when analyzing long image series. Here, we present unDrift, an easy-to-use scientific software for fast and reliable drift correction of SPM images. unDrift provides three different algorithms to determine the drift velocity based on two consecutive SPM images. All algorithms can drift-correct the input data without any additional reference. The first semi-automatic drift correction algorithm analyzes the different distortion of periodic structures in two consecutive up and down (down and up) images, which enables unDrift to correct SPM images without stationary features or overlapping scan areas. The other two algorithms determine the drift velocity from the apparent movement of stationary features either by automatic evaluation of the cross-correlation image or based on positions identified manually by the user. We demonstrate the performance and reliability of unDrift using three challenging examples, namely images distorted by a very high drift velocity, only partly usable images, and images exhibiting an overall weak contrast. Moreover, we show that the semi-automatic analysis of periodic images can be applied to a long series containing hundreds of images measured at the calcite–water interface.

Silver iodide (AgI) particles are known for their outstanding ice nucleation ability. The effective ice nucleation has been explained by the structural similarity of the AgI surfaces and the basal plane of ice I h . However, the relevant AgI surfaces are polar, i.e., thermodynamically instable. This fact implies the existence of a stabilization mechanism. The nature of this stabilization mechanism remains, however, unknown. Additionally, calculations suggest that exclusively the silver‐terminated and not the iodine‐terminated surfaces nucleate ice. So far, no atomically resolved images at any AgI–water interface exist. This is most likely due to the delicate synthesis and the photolytic degradation of AgI. Here, atomic‐resolution images of the AgI–water interface of freshly cleaved crystals are provided. Most importantly, the atomic structure of these surfaces closely resembles the bulk‐truncated geometry, regardless of the termination. In an attempt to reduce ion adsorption, crystals cleaved in ethanol are investigated. Also in this case, a bulk‐truncated geometry is obtained. The measurements thus reveal no indication for a reconstruction of the AgI cleavage planes neither in water nor in ethanol. This study provides atomic‐scale insights into the interfacial structure, which is essential for understanding the excellent ice nucleation ability of AgI.

Mobilisierung beim Abkühlen Auf einer Kupferoberfläche adsorbierte Dimolybdäntetraacetat-Moleküle bilden bei Raumtemperatur eine geordnete Struktur aus aufrecht stehenden Molekülen. Beim Abkühlen auf 220 K werden die Moleküle auf der Oberfläche beweglich. Dieser kontraintuitive Phasenübergang von einer geordneten Struktur bei hohen Temperaturen zu einer mobilen Phase bei tiefen Temperaturen lässt sich durch eine geringere molare Entropie in der mobilen Phase im Vergleich zur geordneten Phase erklären, wie Angelika Kühnle et al. in ihrer Zuschrift auf S. 19265 berichten.

Mobile molecules on surfaces can arrange into stripes due to directional attractive interactions such as p-p stacking, hydrogen, or covalent bonding. The structural arrangement of the stripes depends on the underlying substrate lattice and omnipresent long-range electrostatic interactions. To model the impact of the interplay of short-range attractive and long-range interactions on the molecular arrangements, we study a coarse-grained theoretical approach, where the attractive interaction is described by an anisotropic Ising model. As for the long-range electrostatic interaction, we focus on repulsive dipole-dipole interactions. An efficient Monte Carlo algorithm is developed by which even stripe patterns with very long stripes can be equilibrated. Using this algorithm, we assess the limits of a previously developed mean-field theory, which provides analytical predictions for stripe-to-stripe distance and stripe length distributions. This theory allows one to extract interaction parameters by fitting respective distributions to experimental data. We determine the limits of the applicability of the mean-field theory and beyond its limits suggest a combined approach of mean-field analysis and simulations. The power of this approach is demonstrated by applying it to experimental observed stripe pattern of 3-hydroxybenzoic acid (3-HBA) on the calcite (10.4) surface.