Article pubs.acs.org/JPCC
H2O on Graphene/Ir(111): A Periodic Array of Frozen Droplets Sebastian Standop, Thomas Michely, and Carsten Busse* II. Physikalisches Insitut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany ABSTRACT: Scanning tunneling microscopy (STM) and thermal desorption spectroscopy (TDS) show that deposition of water molecules onto epitaxial graphene on Ir(111) leads to the formation of an extended and well ordered array of amorphous water clusters. We trace the evolution of this cluster phase as dependent on water exposure and deposition temperature. The formation of separated clusters is due to binding energy differences within the moiré superstructure.
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INTRODUCTION Two-dimensional materials are characterized by the contrast between strong covalent interaction in one plane and low ability for bonding perpendicular to it. In consequence, when a two-dimensional material is placed on another one or rests on a substrate, it cannot relax its lattice to achieve an epitaxial match with a small unit cell. Instead, large moiré superstructures result with a periodic variation in the interaction between the upper and the lower layer. Examples are epitaxial graphene (gr)1 or hexagonal boron nitride (hBN)2 on top of metal surfaces, stacks of gr/hBN,3 or bilayers of different transition metal dichalcogenides such as MoS2/MoSe2.4 The locally varying interaction of the two-dimensional material with the underlying substrate brings about spatially varying properties. To give some examples, for the case of epitaxial graphene, a periodicity in the distance between carbon atoms and the substrate,5,6 work function,7 binding energy of image potential states,8,9 or the formation energy of defects10 have been found. An especially intriguing and potentially useful consequence is the modulation in chemical binding as it can lead to the formation of periodic lattices of adsorbates. For the specific example of gr/Ir(111), patterned adsorption has been observed for a wide range of species, ranging from covalently bound metal clusters11 or patches of covalently bound hydrogen atoms12 to large electron acceptor molecules13 bound by van der Waals interaction combined with electrostatic forces. Covalent bonding of adsorbates is enabled by a local rehybridization of the graphene sheet (sp2 → sp3) in regions where this is geometrically enabled by the presence of a metal atom directly underneath a carbon atom.11 Especially with respect to water, adsorption on a substrate with locally varying adsorption energy has exciting consequences. Lenz and Limpowsky14 calculated that for a surface consisting of periodically arranged hydrophilic patches in a hydrophobic matrix, a morphological transition between an ordered array of water droplets and an extended wetting layer © 2014 American Chemical Society
takes place with increasing coverage. This effect was put to use in a pattern of hydrophilic stripes that can act as microchannels.15 The adsorption energy, Eads, of water on free-standing graphene is rather low (a recent density functional theory study yielded Eads ≈ 140 meV16) as the formation of hydrogen bonds between H2O molecules and the carbon sheet is not possible and the only remaining interaction is via the van der Waals force. For the case of graphene on Pt(111), this even induces a novel molecular structure: For a coverage of 2 ML, two parallel flat layers form with hydrogen bonds between the layers and no hydrogen bond donors or acceptors available toward the substrate or the vacuum.17 This structure is metastable with respect to the standard hexagonal ice. Such a metastable bilayer has also been observed on the weakly interacting substrate Au(111).18,19 As the van der Waals interaction is quite long-ranged, the binding of water to graphene can be strongly enhanced by the presence of an underlying metal substrate.16 Although there is a debate regarding the magnitude of this enhancement,20−22 the most recent works imply that graphene is at least translucent with respect to the H2O−substrate interaction.22 In such a scenario, the binding energy of water molecules to epitaxial graphene will strongly depend on the graphene−substrate separation. As this separation varies in graphene moiré structures, a variation in the binding energy can be induced.16 In this work, we observe the formation of an ordered array of water clusters after deposition of H2O onto gr/Ir(111) at low temperatures using scanning tunneling microscopy (STM). With thermal desorption spectroscopy (TDS), we track the conversion of this cluster phase into an extended water layer as a function of deposition temperature and deposited amount. Received: October 7, 2014 Revised: December 22, 2014 Published: December 24, 2014 1418
DOI: 10.1021/jp510140a J. Phys. Chem. C 2015, 119, 1418−1423
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The Journal of Physical Chemistry C
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EXPERIMENTAL SECTION
Our experiments were performed in an ultrahigh vacuum system (base pressure p < 1 × 10−10 mbar). We prepared the substrate by cycles of 5 keV Xe+ bombardment and flashing to 1400 K. Graphene was grown from ethylene (C2H4) up to full monolayer coverage utilizing a combination of temperatureprogrammed growth (TPG, with temperature of adsorption Tads = 300 K, heating temperature Theat = 1350 K) and chemical vapor deposition (CVD) at TCVD = 1125 K.23 Right before water dosing, the crystal was briefly heated to 630 K in order to desorb adsorbates captured during cooling. We used ultrapure water, which was thoroughly Ar-bubbled and purified further by pump−freeze−thaw cycles. Water vapor was dosed through a gold-plated stainless steel tube ending in close proximity to the sample. The rather large diameter of this tube (9 mm) is a compromise between homogeneous exposure of the sample and low total gas load. The sample was exposed to water with a rate of R ≈ 10−2 ML/s at temperatures in the range of 20 K ≤ T ≤ 140 K and subsequently annealed. Water coverages were determined using the integrated thermal desorption spectra and are given with respect to the saturated H2O monolayer on Ir(111), which is a √37superstructure24 similar to the one found for Pt(111)25−27 containing 0.7 H2O molecules per Ir(111) surface atom. In consequence, 1 ML is equivalent to 1.10 × 1019 molecules/m2, which also corresponds to the areal density of molecules in the (0001) plane of ice Ih (1.14 × 1019 molecules/m2). STM measurements were performed with a variabletemperature instrument. The sample was connected to a Heflow cryostat via a flexible copper braid, allowing for sample temperatures down to 20 K. The bias voltage U was applied to the sample, leading to a tunneling current I. The software WSxM28 was used for image processing. Thermal desorption spectroscopy was performed using a quadrupole mass spectrometer equipped with a Feulner cup ending in close proximity to the sample. A linear heating ramp of β = 1 K/s was applied. For data analysis, the linear fit of the T(t) curve was used to compensate for errors of digitalization in the temperature measurement. Furthermore, a background of the mass spectrometer signal was removed, which consisted of two constant contributions (a lower one before the main desorption feature and a higher one afterward) which were joined by a smooth step function.
Figure 1. STM topograph of the sample after adsorption of 3.0 ML H2O on graphene/Ir(111) at Tads = 20 K and 60 s annealing at 80 K. Image width 1100 Å, U = 3.0 V, I = 3 pA. Inset: 2.0 ML of H2O, unit cell of gr/Ir(111) moiré structure and regions of high symmetry are indicated. Image width 130 Å, U = 1.0 V, I = 25 pA.
inset: In the top region, the carbon ring is centered above an atom of the metal surface, whereas in the hcp (fcc) region, the center of the ring is situated above a 3-fold hollow site of hcp (fcc) type.11 The identification of these regions is based on the known appearance of the moiré29 and the known azimuthal orientation of our Ir(111) sample. It is obvious that the clusters are always found in the hcp region. Most clusters show a characteristic depression in the center that we attribute to details of the STM imaging process for these insulating entities rather than to their actual shape. We did not observe that water causes graphene to rupture or intercalates between the carbon sheet and its substrate, as it was found in the related system H2O/gr/Ru(0001),30 especially in the vicinity of graphene defects. We attribute this to the much lower density of such defects for the case of gr/Ir(111) and a lower reactivity of Ir(111) with respect to H2O. We note that even the lowest tunneling currents accessible in our setup (I < 10 pA) induced changes in the morphology of H2O/gr/Ir(111). For comparison, in the systems H2O/hBN/ Rh(111)31 or H2O/Pt(111)27 molecular resolution could be obtained for tunneling currents as high as 100 pA. The induced changes range from picking up single molecules to removing the significant parts of the adsorbate layer in the field of view; see Figure 2 for an example. The manipulation can be due to a direct interaction between the tip and the water molecules or a result of the tunneling current through the poorly conductive water layer. The underlying physical reason is the weak interaction between water molecules and graphene. Figure 3a shows the desorption of H2O from gr/Ir(111) as measured with TDS. For low coverages of Θ0 ≤ 2.55 ML the desorbing species shows a single peak (labeled peak A) with a common leading edge for all coverages. Accordingly, the peak temperature shifts to higher values with increasing initial coverage. For intermediate coverages (3.25 ML ≤ Θ0 ≤ 6.85 ML) we observe a different peak B at a higher temperature, which also has a common leading edge. The transition between peak A and B happens rather abruptly, only for 2.95 ML ≤ Θ0 ≤ 3.15 ML a superposition of both peaks is visible. The STM
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RESULTS AND DISCUSSION An STM image of the sample surface after adsorption of 3.0 ML H2O is shown in Figure 1. One can discern regions containing well separated protrusions, patches of a more continuous layer (e.g., in the center of the image), and bare graphene (we attribute the presence of the bare substrate to manipulation during imaging, see below). Under the tunneling conditions applied here, the clusters have a mean apparent height of (1.0 ± 0.2) Å and the extended phase of (1.3 ± 0.2) Å. For lower coverages (Θ0 ≤ 2 ML), the continuous layer was not observed. The protrusions can be analyzed in experiments with reduced coverages (inset of Figure 1). Under the tunneling conditions present here, both the well-known moiré structure of gr/ Ir(111)29 (imaged in reverse contrast), and well separated bright protrusions can be observed, which we interpret as clusters of H2O molecules. We define regions of high symmetry in the unit cell of the moiré superstructure as indicated in the 1419
DOI: 10.1021/jp510140a J. Phys. Chem. C 2015, 119, 1418−1423
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peak B as a pronounced shoulder. No peaks other than A, B, and C were present up to room temperature. To quantitatively evaluate the desorption spectra we make use of an approximative method: As both the common leading edge and the shift of the peak temperature with initial coverage hint for zero order desorption kinetics, the desorption energy Edes and the frequency factor ν0 can be determined via a fit of the linear regime in an Arrhenius plot. The analysis results in desorption energies of Edes,A = (424 ± 2) meV, Edes,B = (466 ± 2) meV, and Edes,C = (525 ± 2) meV for the respective peaks (errors from the fitting procedure), also listed in Table 1 together with the corresponding values of the frequency factors.
Figure 2. (a), (c) Subsequently recorded STM topographs of approximately 4.0 ML H2O adsorbed on graphene at Tads = 20 K and 60 s annealing at 80 K. Image width: 1000 Å; imaging parameters: U = 170 mV, I = 15 pA. (b), (d) Sketches of the corresponding topographs. Dark: bare graphene, red: water layer, white lines: substrate step edges.
Table 1. Experimentally Determined Desorption Energy Edes and Frequency Factor ν0 for the Different Desorption Peaks A, B, and C peak
coverage
Edes (meV)
ν0 (ML/s)
A B C
Θ0 ≲ 3 ML Θ0 ≳ 3 ML Θ0 ≫ 10 ML
424 ± 2 466 ± 2 525 ± 2
0.6 × 1014±0.1 8.8 × 1014±0.1 3.9 × 1016±0.1
We interpret our thermal desorption spectra starting with peak C. The desorption barrier Edes,C = 525 meV of this peak agrees very well with the theoretical value (based on the interpolation of thermodynamical data) for the enthalpy of sublimation of crystalline ice at 160 K (ΔH = 526 meV)32 and is inside the range of experimental values obtained for the desorption from crystalline ice in earlier studies (Edes,cryst = 500 meV,33 Edes,cryst = 579 meV,34 Edes,cryst = 494 meV35). Hence, we propose that the thick H2O films on graphene are present as crystalline ice at the desorption temperature. A lowered desorption barrier as we observe here for peak B in the case of intermediate coverages is commonly attributed to the presence of amorphous solid water, where the arrangement of water molecules is less ordered. Quantitatively, values of Edes,amorph = 486 meV,33 Edes,amorph = 565 meV,34 and Edes,amorph = 481 meV35 have been reported. Our value of Edes,B = 466 eV is somewhat lower, however it lies in the nature of amorphous materials that their properties can show some spread, depending for example on the average coordination number. The fact that the peak B is still present as a shoulder for the case of high coverages is usually interpreted as follows: Water adsorbed at low temperatures is trapped in an amorphous state with not enough thermal activation available to promote crystallization. Only upon heating does enough energy become available for homogeneous nucleation of crystalline ice, however, then sublimation already sets in, leading to a crystallization during desorption, which causes the shoulder in the desorption spectra (see e.g., refs 34 and 36 where virtually identical curves were measured for the case of H2O/ Pt(111)). For very thin films, desorption is already complete before the recrystallization process is enabled. This model for peaks B and C also explains the zero order of the desorption, as in a multilayer film the number of molecules available for direct desorption does not depend on coverage. The species leaving the surface with peak A has an even lower barrier against desorption. The fact that for coverages larger than 2.55 ML the area of peak A shrinks and peak B starts to grow indicates that the structure causing peak A is only metastable with respect to the structure responsible for peak B. A similar behavior has been observed for the related system gr/ Pt(111) and was also explained by the presence of a metastable
Figure 3. (a) TDS spectra of H2O adsorbed on a graphene-covered Ir(111) surface at Tads = 20 K. The temperature ramp while recording was set to 1 K/s. Θ0 = 1.35−42.80 ML, as indicated in the diagram. (b) TDS spectra of 2.15 ML H2O adsorbed on a graphene covered Ir(111) surface at variable adsorption temperature. The temperature ramp while recording was set to 1 K/s. Tads = 20−130 K, as indicated in the diagram. All spectra for Tads ≤ 90 K are nearly identical, so only one representative spectrum is shown. Pronounced peaks are labeled A, B, and C.
image presented in Figure 1 shows the sample morphology in this transition region. For the highest coverages investigated (Θ0 ≈ 40 ML) the desorption spectrum consists of a single peak C, again shifted to higher temperatures, which contains 1420
DOI: 10.1021/jp510140a J. Phys. Chem. C 2015, 119, 1418−1423
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The Journal of Physical Chemistry C structure for low initial coverages.35 Guided by our STM experiments, we attribute this peak A observed for low coverages to desorption from the cluster phase, and consequently peak B observed for higher coverages to desorption from a more extended layer. Both types of structures are visible in the STM topograph of the intermediate regime, see Figure 1 b. The significantly lower desorption barrier of the clusters can be explained by the large number of undercoordinated molecules in such nanostructures. The fact that we observe zero order is due to the fact that the surface area of the clusters stays more or less constant during the desorption process, at least for large clusters. This indicates that no major reshaping processes are activated, in agreement with the observation of an amorphous phase for even higher temperatures. For the system H2O/gr/Pt(111) where remarkably similar desorption spectra have been observed, the metastable phase has been attributed to an extended amorphous layer, which then transforms into the metastable bilayer introduced above.17−19 Also for our system it could be possible that for a narrow range of initial coverages the cluster phase is succeeded by this bilayer phase, before the bulk crystal structure is obtained. However, we do not have any experimental evidence for this, and there is no indication for an additional phase transformation in our spectra. We rationalize the formation of small clusters as follows: After arriving at the surface from the gas phase, the individual water molecules undergo diffusion on the surface, until they are trapped in a region of maximum binding energy within the moiré (we will discuss potential origins of a local variation of the binding energy below). For effective trapping in the case of significant diffusion, rate equation theory predicts the formation of one nucleus per trap37 consistent with our observation of one cluster per moiré cell. Continued deposition leads to the growth of the clusters. The clusters will be amorphous yet not completely disordered, as molecular processes that can improve order are already activated at low temperatures. For example, the energy barrier to transform a pentagon with an attached monomer into a hexagon was calculated to be only 100 meV.38 In our model, the cluster formation is due to kinetic limitations during growth and not the energetic ground state, as in the model of Lenz et al.14 The clusters continue to grow (e.g., for 2 ML the average size of a cluster is ≈120 molecules) until each moiré cell is full and coalescence sets in. In a simple geometrical model where we assume the clusters to be hemispheres for lack of better knowledge (90° is the contact angle between water and graphite22), coalescence would occur at a critical coverage of Θcrit = 2.3 ML, in reasonable agreement with our experimental observation of a phase transition starting for Θ0 > 2.55 ML. The coexistence of peaks A and B for a small range of coverage is then interpreted as the coexistence of the cluster phase and an extended phase, as imaged in Figure 1b. Upon coalescence, new bonds in the water layer are formed and edge atoms (at the contact of the clusters with gr) disappear. Thereby the H2O layer is stabilized and the desorption barrier increases. To check our model of cluster formation by nucleation in the presence of traps, we increased the deposition temperature. Theory predicts that once enough thermal energy is available to leave a trap, the nucleation density drops significantly,37 leading to the subsequent growth of much larger entities. The corresponding desorption spectra following deposition of Θ0 ≈ 2.15 ML (i.e., significantly below the phase transition) at
different temperatures (Tads = 20−130 K) are depicted in Figure 3b. For Tads ≤ 100 K, the TDS spectra (black and brown curve) show the characteristics already discussed for the spectra shown in Figure 3a: A single peak A with a high temperature tail around 155 K. Water adsorption at Tads = 110 K results in a distinct change of the desorption spectrum, as the intensity of peak A is reduced in favor of the high temperature shoulder (peak B). This intensity shift continues at Tads = 120 K. Here the dominant fraction of the water adsorbates is present in form of an extended layer. Further increasing the adsorption temperature to Tads = 130 K finally leads to competing water desorption. Accordingly, the coverage of the water adlayer is strongly reduced. In conclusion, the outcome of this test confirms our assumption that the formation of clusters is due to the presence a corrugated moiré potential, which hinders diffusion at low temperatures but not at elevated T. Finally, let us discuss potential origins for a preference of the H2O molecules for specific regions of the moiré. The main interaction between H2O and graphene is the van der Waals interaction, enhanced by the polar nature of water.16 Furthermore, the adsorption energy of H2O on graphene can be increased by 30% due to the additional interaction with graphene’s substrate which is in close proximity, an effect that has been called “wetting translucency”. In simple terms, the graphene acts mainly as a spacer layer that keeps the water molecules separated from the more strongly interacting metal substrate.20,22 The locally varying interaction between the carbon sheet and the layer underneath induces a significant height variation of graphene,6 which in turn leads to a variation in the H2O adsorption energy. Further ingredients for trapping of water molecules can be the locally varying work function, which is minimal in the hcp and fcc regions.7 As typically graphene donates (negative) charge to adsorbed water,39 areas with a reduced work function are energetically favorable. On top of that, the local variations in electron density present in the moiré5 induce lateral electric field which can guide and trap polar molecules, as has been observed for large organic molecules on epitaxial graphene.40 We speculate that the variation in gr−Ir(111) distance alone may be already sufficient to explain the preferential adsorption site of water molecules in this system. A closely related effect has been found for H2O/hBN/ Rh(111), where crystalline ice was observed in the valleys of the superperiodic structure of hBN.31 In contrast to the results presented there, we do not observe irregular protrusions in scan profiles through images like the one shown in Figure 1 that could be attributed to a diluted admolecule gas. For a related system (gr/Ru(0001)) a difference in adsorption energy ΔEb = 10 meV between distinct regions of the moiré has been determined for Xe using TDS,2 which is a comparable adsorbate in the sense that it can only interact with the surface via the van der Waals interaction. For water adsorption on epitaxial graphene on a different substrate (gr/Ni(111)), a value of ΔEB = 25 meV was calculated using DFT including van der Waals by Li et al.16 Here, it was already postulated that the preference of H2O for a specific region could lead to the selfassembly into ordered patterns, as observed by us. To sum up, water molecules on gr/Ir(111) experience a periodic modulation of binding energy which can lead to the formation of a well ordered lattice of individual clusters. The system gr/Ir(111) can thus be viewed as a nanoscale realization of a pattern of hydrophilic patches in a hydrophobic matrix.14 1421
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(10) Standop, S.; Lehtinen, O.; Herbig, C.; Lewes-Malandrakis, G.; Craes, F.; Kotakoski, J.; Michely, T.; Krasheninnikov, A. V.; Busse, C. Ion Impacts on Graphene/Ir(111): Interface Channeling, Vacancy Funnels, and a Nanomesh. Nano Lett. 2013, 13, 1948−1955. (11) N’Diaye, A. T.; Gerber, T.; Busse, C.; Mysliveček, J.; Coraux, J.; Michely, T. A Versatile Fabrication Method for Cluster Superlattices. New J. Phys. 2009, 11, 103045. (12) Balog, R.; Jørgensen, B.; Nilsson, L.; Andersen, M.; Rienks, E.; Bianchi, M.; Fanetti, M.; Lægsgaard, E.; Baraldi, A.; Lizzit, S. Bandgap Opening in Graphene Induced by Patterned Hydrogen Adsorption. Nat. Mater. 2010, 9, 315−319. (13) Barja, S.; Garnica, M.; Hinarejos, J. J.; Vázquez de Parga, A. L.; Martn, N.; Miranda, R. Self-Organization of Electron Acceptor Molecules on Graphene. Chem. Commun. 2010, 46, 8198−8200. (14) Lenz, P.; Lipowsky, R. Morphological Transitions of Wetting Layers on Structured Surfaces. Phys. Rev. Lett. 1998, 80, 1920−1923. (15) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Liquid Morphologies on Structured Surfaces: From Microchannels to Microchips. Science 1999, 283, 46−49. (16) Li, X.; Feng, J.; Wang, E.; Meng, S.; Klimeš, J.; Michaelides, A. Influence of Water on the Electronic Structure of Metal-Supported Graphene: Insights From Van Der Waals Density Functional Theory. Phys. Rev. B 2012, 85, 085425. (17) Kimmel, G. A.; Matthiesen, J.; Baer, M.; Mundy, C. J.; Petrik, N. G.; Smith, R. S.; Dohnalék, Z.; Kay, B. D. No Confinement Needed: Observation of a Metastable Hydrophobic Wetting Two-Layer Ice on Graphene. J. Am. Chem. Soc. 2009, 131, 12838−12844. (18) Stacchiola, D.; Park, J. B.; Liu, P.; Ma, S.; Yang, F.; Starr, D. E.; Muller, E.; Sutter, P.; Hrbek, J. Water Nucleation on Gold: Existence of a Unique Double Bilayer. J. Phys. Chem. C 2009, 113, 15102− 15105. (19) Corem, G.; Kole, P. R.; Zhu, J.; Kravchuk, T.; Manson, J. R.; Alexandrowicz, G. Ordered H2O Structures on a Weakly Interacting Surface: A Helium Diffraction Study of H2O/Au(111). J. Phys. Chem. C 2013, 117, 23657−23663. (20) Rafiee, J.; Mi, X.; Gullapalli, H.; Thomas, A. V.; Yavari, F.; Shi, Y.; Ajayan, P. M.; Koratkar, N. A. Wetting Transparency of Graphene. Nat. Mater. 2012, 11, 217−222. (21) Raj, R.; Maroo, S. C.; Wang, E. N. Wettability of Graphene. Nano Lett. 2013, 13, 1509−1515. (22) Shih, C.-J.; Strano, M. S.; Blankschtein, D. Wetting Translucency of Graphene. Nat. Mater. 2013, 12, 866−869. (23) Van Gastel, R.; N’Diaye, A. T.; Wall, D.; Coraux, J.; Busse, C.; Buckanie, N. M.; Meyer zu Heringdorf, F. J.; Horn-von Hoegen, M.; Michely, T.; Poelsema, B. Selecting a Single Orientation for Millimeter Sized Graphene Sheets. Appl. Phys. Lett. 2009, 95, 121901−121901. (24) Standop, S. Water adsorption and Ion Induced Defect Formation: A Comparative Study of Graphene and Noble Metal Surfaces. Ph.D. thesis, University of Cologne, 2013. (25) Standop, S.; Redinger, A.; Morgenstern, M.; Michely, T.; Busse, C. Molecular Structure of the H2O Wetting Layer on Pt (111). Phys. Rev. B 2010, 82, 161412. (26) Nie, S.; Feibelman, P. J.; Bartelt, N. C.; Thürmer, K. Pentagons and Heptagons in the First Water Layer on Pt(111). Phys. Rev. Lett. 2010, 105, 026102. (27) Standop, S.; Morgenstern, M.; Michely, T.; Busse, C. H2O on Pt(111): Structure and Stability of the First Wetting Layer. J. Phys.: Condens. Matter 2012, 24, 124103. (28) Horcas, I.; Fernández, R.; Gómez-Rodrguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (29) N’Diaye, A. T.; Coraux, J.; Plasa, T. N.; Busse, C.; Michely, T. Structure of Epitaxial Graphene on Ir (111). New J. Phys. 2008, 10, 043033. (30) Feng, X.; Maier, S.; Salmeron, M. Water Splits Epitaxial Graphene and Intercalates. J. Am. Chem. Soc. 2012, 134, 5662−5668.
CONCLUSIONS In conclusion, our combined STM and TDS study shows that water molecules deposited onto gr/Ir(111) at low temperatures are kinetically trapped in the hcp regions of the graphene moiré. These regions are preferred as they are closest to the substrate, hence allowing a strong contribution of metal−water interaction to the total binding energy due to the wetting translucency of graphene. Continued growth leads to the formation of an ordered lattice of clusters of amorphous solid water. The clusters finally coalesce once the preferred areas overflow and form an extended amorphous layer. For thick layers, crystallization is possible below the desorption temperature. Growth at elevated temperatures directly leads to the extended phase, as the water molecules are able to diffuse out of their traps.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from the European Commission via the seventh Framework Program project “Graphene for Nanoscaled Applications (GRENADA)” and the Bonn-Cologne Graduate School of Physics and Astronomy.
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REFERENCES
(1) Coraux, J.; Engler, M.; Busse, C.; Wall, D.; Buckanie, N.; Meyer zu Heringdorf, F. J.; van Gastel, R.; Poelsema, B.; Michely, T. Growth of Graphene on Ir (111). New J. Phys. 2009, 11, 023006. (2) Brugger, T.; Günther, S.; Wang, B.; Dil, H.; Bocquet, M.-L.; Osterwalder, J.; Wintterlin, J.; Greber, T. Comparison of Electronic Structure and Template Function of Single-Layer Graphene and Hexagonal Boron Nitride Nanomesh on Ru(0001). Phys. Rev. B 2009, 79, 045407. (3) Xue, J.; Sanchez-Yamagishi, J.; Bulmash, D.; Jacquod, P.; Deshpande, A.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; LeRoy, B. J. Scanning Tunnelling Microscopy and Spectroscopy of Ultra-Flat Graphene on Hexagonal Boron Nitride. Nat. Mater. 2011, 10, 282−285. (4) Kang, J.; Tongay, S.; Li, J.; Wu, J. Monolayer Semiconducting Transition Metal Dichalcogenide Alloys: Stability and Band Bowing. J. Appl. Phys. 2013, 113, 143703. (5) Busse, C.; Lazić, P.; Djemour, R.; Coraux, J.; Gerber, T.; Atodiresei, N.; Caciuc, V.; Brako, R.; N’Diaye, A. T.; Blügel, S. Graphene on Ir (111): Physisorption with Chemical Modulation. Phys. Rev. Lett. 2011, 107, 36101. (6) Runte, S.; Lazić, P.; Vo-Van, C.; Coraux, J.; Zegenhagen, J.; Busse, C. Graphene Buckles under Stress: An X-ray Standing Wave and Scanning Tunneling Microscopy Study. Phys. Rev. B 2014, 89, 155427. (7) Altenburg, S. J.; Berndt, R. Local Work Function and STM TipInduced Distortion of Graphene on Ir(111). New J. Phys. 2014, 16, 053036. (8) Borca, B.; Barja, S.; Garnica, M.; Sánchez-Portal, D.; Silkin, V. M.; Chulkov, E. V.; Hermanns, C. F.; Hinarejos, J. J.; Vázquez de Parga, A. L.; Arnau, A. Potential Energy Landscape for Hot Electrons in Periodically Nanostructured Graphene. Phys. Rev. Lett. 2010, 105, 036804. (9) Armbrust, N.; Güdde, J.; Jakob, P.; Höfer, U. Time-Resolved Two-Photon Photoemission of Unoccupied Electronic States of Periodically Rippled Graphene on Ru (0001). Phys. Rev. Lett. 2012, 108, 56801. 1422
DOI: 10.1021/jp510140a J. Phys. Chem. C 2015, 119, 1418−1423
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The Journal of Physical Chemistry C (31) Ma, H.; Brugger, T.; Berner, S.; Ding, Y.; Iannuzzi, M.; Hutter, J.; Osterwalder, J.; Greber, T. Nano-ice on Boron Nitride Nanomesh: Accessing Proton Disorder. ChemPhysChem 2010, 11, 399−403. (32) Feistel, R.; Wagner, W. Sublimation Pressure and Sublimation Enthalpy of H2O Ice Ih between 0 and 273.16 K. Geochim. Cosmochim. Acta 2007, 71, 36−45. (33) Speedy, R. J.; Debenedetti, P. G.; Smith, R. S.; Huang, C.; Kay, B. D. The Evaporation Rate, Free Energy, and Entropy of Amorphous Water at 150 K. J. Chem. Phys. 1996, 105, 240−244. (34) Smith, R. S.; Matthiesen, J.; Knox, J.; Kay, B. D. Crystallization Kinetics and Excess Free Energy of H2O and D2O Nanoscale Films of Amorphous Solid Water. J. Phys. Chem. A 2011, 115, 5908−5917. (35) Smith, R. S.; Matthiesen, J.; Kay, B. D. Desorption Kinetics of Methanol, Ethanol, and Water from Graphene. J. Phys. Chem. A 2014, 118, 8242−8250. (36) Dohnálek, Z.; Ciolli, R. L.; Kimmel, G. A.; Stevenson, K. P.; Smith, R. S.; Kay, B. D. Substrate Induced Crystallization of Amorphous Solid Water at Low Temperatures. J. Chem. Phys. 1999, 110, 5489−5492. (37) Kotrla, M.; Krug, J.; Šmilauer, P. Submonolayer Epitaxy with Impurities: Kinetic Monte Carlo Simulations and Rate-Equation Analysis. Phys. Rev. B 2000, 62, 2889. (38) Batista, E. R.; Jónsson, H. Diffusion and Island Formation on the Ice Ih Basal Plane Surface. Comput. Mater. Sci. 2001, 20, 325−336. (39) Leenaerts, O.; Partoens, B.; Peeters, F. M. Water on graphene: Hydrophobicity and Dipole Moment Using Density Functional Theory. Phys. Rev. B 2009, 79, 235440. (40) Zhang, H. G.; Sun, J. T.; Low, T.; Zhang, L. Z.; Pan, Y.; Liu, Q.; Mao, J. H.; Zhou, H. T.; Guo, H. M.; Du, S. X.; Guinea, F.; Gao, H.-J. Assembly of Iron Phthalocyanine and Pentacene Molecules on a Graphene Monolayer Grown on Ru(0001). Phys. Rev. B 2011, 84, 245436.
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DOI: 10.1021/jp510140a J. Phys. Chem. C 2015, 119, 1418−1423
