Submonolayer Coverages Research Articles

Ecofriendly Filtration of Silver Nanoparticles for Ultrasensitive Surface-Enhanced (Resonance) Raman Spectroscopy-Based Detection.

In this study, a widely used colloid of Creighton AgNPs (ORI, 1-100 nm, mostly ≤ 40 nm, ∼10 μg mL-1) was rapidly manipulated via tangential flow filtration (TFF) for highly reproducible surface-enhanced (resonance) Raman spectroscopy (SE(R)RS) experiments down to the single-molecule (SM) level. The quasi-spherical AgNPs were size-selected, purified, and concentrated in two TFF fractions of a cutoff diameter of ∼40 nm: AgNP ≤ 40 (∼900 μg mL-1) and AgNP ≥ 40 (∼100 μg mL-1). The SE(R)S-based sensing capabilities of the two TFF fractions were then tested under pre-resonance (632.8 nm) and resonance (532.1 nm) excitation conditions for rhodamine 6G (R6G, 10-6-10-15 M). Both TFF isolates, AgNP ≤ 40 and AgNP ≥ 40, were more effective in adsorbing the R6G analyte (≥91%) than the original colloid (≥78%) at submonolayer coverages. Furthermore, the surface enhancement factors (SEF) of the two TFF fractions were markedly superior to those of ORI under all excitation conditions. SERS at 632.8 nm: only AgNP ≥ 40 enabled the detection of R6G at 10-9 M and produced the largest SEF (2.1 × 106). SE(R)RS and SM-SERRS at 532.1 nm: AgNP ≥ 40 gave rise to the largest SEF values (2.5 × 1010) corresponding to the SM regime down to 10-15 M of R6G. Nevertheless, AgNP ≤ 40 compensated for the size-dependence of the electromagnetic enhancements by an increase in the silver concentration, which led to SEF values comparable to those of AgNP ≥ 40 through additional resonance enhancements. TFF resulted into a ∼100-fold increase (AgNP ≤ 40) in the number of negatively charged AgNPs that were available to electrostatically bridge R6G cations and form SERRS "hot-spots" (AgNP-R6G-AgNP) within the focal volume. Evidently, the interplay between AgNP size, AgNP concentration, and excitation wavelength governs the SE(R)RS enhancements. This study demonstrated that TFF can facilitate the ecofriendly isolation of spherical AgNPs of controlled morphological and plasmonic properties for further enhancing their sensing capabilities as SE(R)RS substrates.

Probing the Roles of Temperature and Cooperative Effects in Chirality-Induced Spin Selectivity: Photoelectron Spin Polarization in Helical Tetrapyrroles.

We investigate the roles of molecular vibrations and intermolecular interactions in the mechanism underlying chirality-induced spin selectivity (CISS) in monolayers of helical tetrapyrrole (TPBT) molecules. The spin polarization of photoelectrons emitted from TPBT-functionalized Cu(111) surfaces was measured as a function of the temperature and the surface coverage. We employed DFT calculations to determine the energy and temperature-dependent population of vibrational modes which vary either the molecular pitch and/or the molecular radius. In combination, the data demonstrate that molecular vibrations do not play a significant role for CISS in the TPBT layers. Submonolayer coverages were created by gradual thermal desorption of the molecules from the surface during the spin polarization measurements. While the spin polarization scales nonlinearly with the surface coverage, this behavior can be rationalized entirely through changes of the photoelectron yield upon surface functionalization, and therefore represents no evidence for cooperative effects involved in CISS.

Growth and Ag-encapsulation of Pt islands on Ag(111) at room temperature

The growth of Pt islands at submonolayer coverages on Ag(111) at room temperature were investigated with scanning tunneling microscopy. A two-step mechanism for growth of the islands is proposed. First, Pt replaces Ag substrate atoms through a place-exchange process. Next, Pt adatoms nucleate at substitutional Pt sites and Pt islands subsequently grow from these sites. At room temperature, Ag atoms migrate to cover Pt islands, creating vacancy pits on the terraces and bays on the step edges. Ag atoms nucleate at corner sites of the Pt islands, and the layer of Ag atoms on the Pt islands grow from these sites.

(Invited) Electrochemistry of CdSe Quantum Dots in Organic Solvents. from Stochastic Electrochemistry to Film Behavior

We present our electrochemistry results of CdSe quantum dots in two organic solvents: neat methanol and acetonitrile. In MeOH, the quantum dots (QDs) can be detected through stochastic collisions; that is, as agglomerates of QDs collide with an electrode surface, the photocurrent changes as a result of the CdSe entities that collide with the electrode surface.1 The experiment presented here uses Pt ultramicroelectrodes (UME), that is, electrodes with a diameter of 25 μm or less. Our results show that the CdSe can accumulate on the surface of the electrode, and the technique provides a high degree of control. By controlling the time the electrode collects QDs, one can control the amount of CdSe that accumulates at the electrode surface. After collecting CdSe on the surface, we perform cyclic voltammetry experiments to confirm that the electrode surface has changed. In acetonitrile (MeCN), we can perform CVs over large potential windows to study the electrochemistry of the deposited CdSe. The electrochemical response of the deposited CdSe resembles that of a film, although the surface has sub-monolayer coverages, as will be discussed from the images of scanning electron microscopy (SEM).Figure 1 shows an example of a LSV in a region of interest where the current due to the CdSe significantly deviates from the background current. The LSV was run in the dark, so the current in the figure is a function of the density of states in the CdSe structures. It is also a function of the reversibility of the CV, i.e., mainly of the scan rate of the experiment. After the CV experiments, the Pt UME can be imaged in the SEM to correlate the electrochemical results with the CdSe deposited on the electrode surface. We will discuss our findings in terms of the results in the literature on CdSe electrochemistry (e.g., refs 2-5). References. (1) Subedi, P.; Parajuli, S.; Alpuche-Aviles, M. A., "Single Entity Behavior of CdSe Quantum Dot Aggregates During Photoelectrochemical Detection", Frontiers in Chemistry 9 (2021) DOI: 10.3389/fchem.2021.733642(2) Kohl, P. A.; Bard, A. J., "Semiconductor electrodes. 13. Characterization and behavior of n-type zinc oxide, cadmium sulfide, and gallium phosphide electrodes in acetonitrile solutions", Journal of the American Chemical Society 99, 7531-7539 (1977) DOI: 10.1021/ja00465a023(3) Hines, M. A.; Guyot-Sionnest, P., "Synthesis and characterization of strongly luminescing ZnS-capped CdSe nanocrystals", The Journal of Physical Chemistry 100, 468-471 (1996) DOI: 10.1021/jp9530562(4) Zhang, J. Z., "Interfacial Charge Carrier Dynamics of Colloidal Semiconductor Nanoparticles", The Journal of Physical Chemistry B 104, 7239-7253 (2000) DOI: 10.1021/jp000594s(5) Myung, N.; Ding, Z.; Bard, A. J., "Electrogenerated chemiluminescence of CdSe nanocrystals", Nano Letters 2, 1315-1319 (2002) Figure 1

(η8-Cyclooctatetraene)(η5-fluorenyl)titanium: a processable molecular spin qubit with optimized control of the molecule-substrate interface.

Depositing single paramagnetic molecules on surfaces for sensing and quantum computing applications requires subtle topological control. To overcome issues that are often encountered with sandwich metal complexes, we exploit here the low symmetry architecture and suitable vaporability of mixed-sandwich [FluTi(cot)], Flu = fluorenyl, cot = cyclooctatetraene, to drive submonolayer coverage and select an adsorption configuration that preserves the spin of molecules deposited on Au(111). Electron paramagnetic resonance spectroscopy and ab initio quantum computation evidence a d z 2 ground state that protects the spin from phonon-induced relaxation. Additionally, computed and measured spin coherence times exceed 10 μs despite the molecules being rich in hydrogen. A thorough submonolayer investigation by scanning tunneling microscopy, X-ray photoelectron and absorption spectrocopies and X-ray magnetic circular dichroism measurements supported by DFT calculations reveals that the most stable configuration, with the fluorenyl in contact with the metal surface, prevents titanium(iii) oxidation and spin delocalization to the surface. This is a necessary condition for single molecular spin qubit addressing on surfaces.

The role of chlorine promoters in mediating particle size effects in silver-catalyzed ethylene epoxidation

Measured chlorine coverages over Ag/α-Al2O3 catalysts with varying Ag weight loadings (1.3–35 wt%) and different Ag particle size distributions when parsed in terms of Cl coverages over particles of varying size constituted in the distribution reveal that Ag particles of size below ∼30 nm are covered in more than 1 monolayer (ML) of chlorine in presence of 3.5 ppm C2H5Cl. These assessments of Cl coverages were made possible by correlating the cumulative Cl coverage to the lognormal surface area distribution of 22 Ag/α-Al2O3 samples using a single exponential decay function. Fully chlorided small Ag particles exhibit low rates of epoxidation and only large Ag particles which exhibit sub-monolayer Cl coverages catalyze ethylene epoxidation with high ethylene oxide (EO) rates and selectivity. The Ag particle size dependence of Cl coverages plausibly explains why ≥ 100 nm Ag particles are typical in promoted Ag/α-Al2O3 ethylene epoxidation catalysts.

Effect of Dynamic and Preferential Decoration of Pt Catalyst Surfaces by WOx on Hydrodeoxygenation Reactions.

Catalysts containing Pt nanoparticles and reducible transition-metal oxides (WOx, NbOx, TiOx) exhibit remarkable selectivity to aromatic products in hydrodeoxygenation (HDO) reactions for biomass valorization, contrasting the undesired aromatic hydrogenation typically observed for metal catalysts. However, the active site(s) responsible for the high selectivity remains elusive. Here, theoretical and experimental analyses are combined to explain the observed HDO reactivity by interrogating the organization of reduced WOx domains on Pt surfaces at sub-monolayer coverage. The SurfGraph algorithm is used to develop model structures that capture the configurational space (∼1000 configurations) for density functional theory (DFT) calculations of a W3O7 trimer on stepped Pt surfaces. Machine-learning models trained on the DFT calculations identify the preferential occupation of well-coordinated Pt sites (≥8 Pt coordination number) by WOx and structural features governing WOx-Pt stability. WOx/Pt/SiO2 catalysts are synthesized with varying W loadings to test the theoretical predictions and relate them to HDO reactivity. Spectroscopy- and microscopy-based catalyst characterizations identify the dynamic and preferential decoration of well-coordinated sites on Pt nanoparticles by reduced WOx species, consistent with theoretical predictions. The catalytic consequences of this preferential decoration on the HDO of a lignin model compound, dihydroeugenol, are clarified. The effect of WOx decoration on Pt nanoparticles for HDO involves WOx inhibition of aromatic ring hydrogenation by preferentially blocking well-coordinated Pt sites. The identification of preferential decoration on specific sites of late-transition-metal surfaces by reducible metal oxides provides a new perspective for understanding and controlling metal-support interactions in heterogeneous catalysis.

Underpotential deposition of Cu onto Au(111) in the presence of acetate

The underpotential deposition (UPD) of Cu on Au(111) in the presence of acetate is investigated by cyclic voltammetry, potential step experiments, and in-situ STM imaging. The specific adsorption of acetate stabilizes the UPD monolayer (ML) of Cu on Au(111). Three distinct deposition processes are identified by cyclic voltammetry. The first two deposition processes corresponding to Cu sub-monolayer coverages are reversible. Strong kinetic effects arise for higher Cu coverage during the deposition and dissolution, which gradually lower the reversibility of the electrocrystallization process. Dissolution of the full Cu monolayer is shifted positively by about 0.3 V. In addition, bulk deposition is kinetically hindered for an extended potential region. The acetate coverage on the complete Cu monolayer is estimated to be 0.2–0.3 ML. It is proposed that adsorbed acetate on the Cu UPD ML is related to the kinetic hindrance observed in the experiments. (Surface) alloying of Cu and Au is observed by cyclic voltammetry and in-situ STM.

Light-Induced Increase of the Local Molecular Coverage on a Surface.

Light is a versatile tool to remotely activate molecules adsorbed on a surface, for example, to trigger their polymerization. Here, we explore the spatial distribution of light-induced chemical reactions on a Au(111) surface. Specifically, the covalent on-surface polymerization of an anthracene derivative in the submonolayer coverage range is studied. Using scanning tunneling microscopy and X-ray photoemission spectroscopy, we observe a substantial increase of the local molecular coverage with the sample illumination time at the center of the laser spot. We find that the interplay between thermally induced diffusion and the reduced mobility of reaction products steers the accumulation of material. Moreover, the debromination of the adsorbed species never progresses to completion within the experiment time, despite a long irradiation of many hours.

Evolutionary Optimized, Monocrystalline Gold Double Wire Gratings as a Novel SERS Sensing Platform.

Achieving reliable and quantifiable performance in large-area surface-enhanced Raman spectroscopy (SERS) substrates poses a formidable challenge, demanding signal enhancement while ensuring response uniformity and reproducibility. Conventional SERS substrates often made of inhomogeneous materials with random resonator geometries, resulting in multiple or broadened plasmonic resonances, undesired absorptive losses, and uneven field enhancement. These limitations hamper reproducibility, making it difficult to conduct comparative studies with high sensitivity. This study introduces an innovative approach that addresses these challenges by utilizing monocrystalline gold flakes to fabricate well-defined plasmonic double-wire resonators through focused ion-beam lithography. Inspired by biological strategy, the double-wire grating substrate (DWGS) geometry is evolutionarily optimized to maximize the SERS signal by enhancing both excitation and emission processes. The use of monocrystalline material minimizes absorption losses and ensures shape fidelity during nanofabrication. DWGS demonstrates notable reproducibility (RSD=6.6%), repeatability (RSD=5.6%), and large-area homogeneity >104µm2. It provides a SERS enhancement for sub-monolayer coverage detection of 4-Aminothiophenol analyte. Furthermore, DWGS demonstrates reusability, long-term stability on the shelf, and sustained analyte signal stability over time. Validation with diverse analytes, across different states of matter, including biological macromolecules, confirms the sensitive and reproducible nature of DWGSs, thereby establishing them as a promising platform for future sensing applications.

Transfer doping of epitaxial graphene on SiC(0001) using Cs

Control of the charge carrier concentration is essential for applications of graphene. Here, we demonstrate the doping of epitaxial graphene on SiC(0001) via charge transfer from an adsorbed layer of Cs atoms with sub-monolayer coverage. The electronic structure of the graphene is analyzed using x-ray and angle-resolved photoelectron spectroscopy. In H-intercalated, quasi-freestanding monolayer graphene (QFMLG), the Dirac point can be tuned continuously from p-type to strong n-type doping. For strong n-type doping, analysis of the core level binding energies implies a deviation from a rigid band shift. This might be explained by an increased screening of the atomic core potential due to the higher number of charge carriers per C atom in the graphene layer. Furthermore, charge transfer into the SiC substrate leads to a change in band bending at the SiC/QFMLG interface, which saturates into a flat band scenario at higher Cs coverage. An analysis of the Fermi surfaces suggests an increasing electron-phonon-coupling in strongly doped QFMLG. In monolayer graphene (MLG), which is intrinsically n-type doped due to the presence of the buffer layer at the SiC interface, n-type doping can be enhanced by Cs evaporation in a similar fashion. In contrast to QFMLG, core level spectra and Dirac cone position in MLG apparently show a rigid band shift even for very high doping, emphasizing the importance of the substrate.

Controlling oxide promoter coverage and microstructure on metals of inverse catalysts: Application to liquid phase tetrahydrofurfuryl alcohol conversion to 1,5-pentanediol

Metal M1/metal oxide M2Ox (M1M2Ox) inverse catalysts, where the oxide layer rests atop metal, have gained attention for their distinct catalytic performance. They are intensively studied in biomass upgrading, e.g., the hydrogenolysis of tetrahydrofurfuryl alcohol to produce 1,5-pentanediol. Pt and MOx (M = W, Mo, Re, Nb) exhibit remarkable synergism in activity and selectivity, but the active sites remain poorly understood. Here, we examine the influence of oxide loading on PtMOx inverse catalysts and introduce a high-pressure wash treatment to leach the excess oxide from carbon and optimize their structure. The findings reveal a saturation sub-monolayer MOx coverage with 2D atomic structure on Pt that is crucial for performance; excessive loading leads to nanocrystalline of lower activity, and low loading exposes unselective metal sites. Wash treatment selectively removes MOx from carbon, enhances their dispersion on Pt, and improves, in most cases, the performance. Tuning the inverse structure advances structure-reactivity understanding.

Plasmonic sensing: FDTD calculations to interpret experimental LSPR water adsorption isotherms

Finite-Difference Time-Domain (FDTD) calculations are widely used to interpret experimental Localized Surface Plasmon Resonance (LSPR) spectra. With FDTD it is possible to predict the physical parameters of the nanostructures that will theoretically give the best experimental LSPR signals. This information is crucial for optimizing sample fabrication and saving significant time. FDTD is also essential to control and calibrate various parameters of the nanostructures and to understand effects such as a Cr adhesion layer and LSPR sensor size distribution. Finally, for water adsorption sensing, FDTD confirms that the LSPR peak shift strictly follows Campbell’s model, even at submonolayer coverage.

Tungsten oxide nanomaterial interactions below monolayer coverage

A tungsten oxide load at submonolayer coverage of alumina renders highly dispersed dimeric polytungstate (octahedral and distorted octahedral coordination) and isolated monotungstate (tetrahedral) species. The polytungstate/monotungstate proportion increases when approaching monolayer. Crystalline WO3 nanoparticles emerge at higher loads. All components display hexavalent oxidation; yet, tungsten‐ oxide (W–O–W) and tungsten‐ oxide‐ alumina (W–O–Al) interactions were discerned by X‐ ray photoelectron spectroscopy of the WOx/Al2O3 materials herein prepared with 1, 3, and 5 weight percentages. The 5 wt % sample displayed the highest intensity percentage of W–O–Al species correlated to the highest methanol conversion (45 %), ascribed to the appearance of the weak acidity disclosed by temperature‐ programmed desorption of ammonia.

Binding energies of ethanol and ethylamine on interstellar water ices: synergy between theory and experiments.

Experimental and computational chemistry are two disciplines used to conduct research in astrochemistry, providing essential reference data for both astronomical observations and modeling. These approaches not only mutually support each other, but also serve as complementary tools to overcome their respective limitations. Leveraging on such synergy, we characterized the binding energies (BEs) of ethanol (CH3CH2OH) and ethylamine (CH3CH2NH2), two interstellar complex organic molecules (iCOMs), on crystalline and amorphous water ices through density functional theory (DFT) calculations and temperature-programmed desorption (TPD) experiments. Experimentally, CH3CH2OH and CH3CH2NH2 behave similarly, in which desorption temperatures are higher on the water ices than on a bare gold surface. Computed cohesive energies of pure ethanol and ethylamine bulk structures allow describing of the BEs of the pure species deposited on the gold surface, as extracted from the TPD curve analyses. The BEs of submonolayer coverages of CH3CH2OH and CH3CH2NH2 on the water ices cannot be directly extracted from TPD due to their co-desorption with water, but they are computed through DFT calculations, and found to be greater than the cohesive energy of water. The behaviour of CH3CH2OH and CH3CH2NH2 is different when depositing adsorbate multilayers on the amorphous ice, in that, according to their computed cohesive energies, ethylamine layers present weaker interactions compared to ethanol and water. Finally, from the computed BEs of ethanol, ethylamine and water, we can infer that the snow-lines of these three species in protoplanetary disks will be situated at different distances from the central star. It appears that a fraction of ethanol and ethylamine is already frozen on the grains in the water snow-lines, causing their incorporation in water-rich planetesimals.

Transition mechanism of the coverage-dependent polymorphism of self-assembled melamine nanostructures on Au(111).

Molecular self-assembled films have recently attracted increasing attention within the field of nanotechnology as they offer a route to obtain new materials. However, careful selection of the molecular precursors and substrates, as well as exhaustive control of the system evolution is required to obtain the best possible outcome. The three-fold rotational symmetry of melamine molecules and their capability to form hydrogen bonds make them suitable candidates to synthesize this type of self-assembled network. In this work, we have studied the polymorphism of melamine nanostructures on Au(111) at room temperature. We find two coverage-dependent phases: a honeycomb structure (α-phase) for submonolayer coverage and a close-packed structure (β-phase) for full monolayer coverage. A combined scanning tunnel microscopy and density functional theory based-calculations study of the transition regime where both phases coexist allows describing the mechanism underlying this coverage driven phase transition in terms of the changes in the molecular lateral tension.

Local surface dynamics in the adsorbed film of pentane isomers on graphite

Surface diffusion due to molecular motion within adsorbed fluid films can be a dominant contributor to mass transfer in porous materials. In micro- and mesopores, surface diffusion is classified into three regimes: the monolayer, multilayer, and condensed phase. Although the dominant molecular motion is restricted to the in-plane direction for sub-monolayer coverage, it is more complicated within the multilayer adsorbed phase, especially at the interface between the vapour phase and the adsorbed phase. We have performed molecular dynamic simulations to study the surface self-diffusion and the collective diffusion of pentane isomers on graphite surface. In the bulk liquid, linear n-pentane has higher self-diffusivity than neopentane at the same temperature. The contrary is observed in an adsorption system where neopentane consistently shows a greater lateral self-diffusion coefficient. In the multilayer regime, we show that the contribution from the vapour phase to the global diffusion is significant. In agreement with the trend observed for self-diffusivities, the collective diffusivities of both isomers show distinct behaviours, particularly in the multilayer regimes. This study highlights the complicated diffusion behaviour of multilayered molecules and the significant contribution from the vapour phase to the dynamics at high pressures.

Effect of a Single Platinum Atom within a Small Metal Oxide Cluster: Reaction of DMMP with Size-Selected Pt1Zr2O7 Supported on HOPG.

Chemical warfare agents (CWAs) are a persistent threat facing civilians and military personnel across the modern geopolitical landscape. The development of the next generation of protective and sensing materials stands to benefit from an improved fundamental understanding of the interaction of CWA molecules with the active components of such candidate materials. The use of model systems in well-controlled environments offers a route to glean such information and has been applied here to investigate the fundamental interaction of a nerve agent simulant molecule, dimethyl methylphosphonate (DMMP), with a small cluster model of a single atom catalyst (SAC) active site. The cluster models, Pt1Zr2O7, were prepared by depositing mass-selected cluster anions synthesized in the gas phase onto a 100 K highly oriented pyrolytic graphite (HOPG) substrate surface prepared in ultra-high vacuum (UHV) at sub-monolayer coverage. Upon deposition, the cluster anions lost their charge to the electrically conductive surface to yield free-standing neutral clusters. The HOPG-supported clusters were characterized by X-ray photoelectron spectroscopy (XPS) to determine the oxidation states and chemical environment of the metal atoms present within the clusters. The reactivity of the clusters with DMMP was investigated via temperature-programmed desorption/reaction (TPD/R) and XPS experiments in which the clusters were exposed to DMMP and incrementally heated to higher temperatures. In contrast to two other HOPG-supported clusters, (ZrO2)3 and Pt1Ti2O7, recently investigated in our laboratory, Pt1Zr2O7 decomposed DMMP to primarily evolve a methane species, which was completely absent for the other clusters.

Voltage-Dependent FTIR and 2D Infrared Spectroscopies within the Electric Double Layer Using a Plasmonic and Conductive Electrode.

Strong electric fields exist between the electric double layer and charged surfaces. These fields impact molecular structures and chemistry at interfaces. We have developed a transparent electrode with infrared plasmonic enhancement sufficient to measure FTIR and two-dimensional infrared spectra at submonolayer coverages on the surface to which a voltage can be applied. Our device consists of an infrared transparent substrate, a 10-20 nm layer of conductive indium tin oxide (ITO), an electrically resistive layer of 3-5 nm Al2O3, and a 3 nm layer of nonconductive plasmonic gold. The materials and thicknesses are set to maximize the surface number density of the monolayer molecules, electrical conductivity, and plasmonic enhancement while minimizing background signals and avoiding Fano line shape distortions. The design was optimized by iteratively characterizing the material roughness and thickness with atomic force microscopy and electron microscopy and by monitoring the plasmon resonance enhancement with spectroscopy. The design is robust to repeated fabrication. This new electrode is tested on nitrile functional groups using a monolayer of 4-mercaptobenzonitrile as well as on CO and CC stretching modes using 4-mercaptobenzoic acid methyl ester. A voltage-dependent Stark shift is observed on both monolayers. We also observe that the transition dipole strength of the CN mode scales linearly with the applied voltage, providing a second way of measuring the surface electric field strength. We anticipate that this cell will enable many new voltage-dependent infrared experiments under applied voltages.

Interface versus Bulk Light-Induced Switching in Spin-Crossover Molecular Ultrathin Films Adsorbed on a Metallic Surface.

Spin-crossover molecules present the unique property of having two spin states that can be controlled by light excitation at low temperature. Here, we report on the photoexcitation of [FeII((3, 5-(CH3)2Pz)3BH)2] (Pz = pyrazolyl) ultrathin films, with thicknesses ranging from 0.9 to 5.3 monolayers, adsorbed on Cu(111) substrate. Using X-ray absorption spectroscopy measurements, we confirm the anomalous light-induced spin-state switching observed for sub-monolayer coverage and demonstrate that it is confined to the first molecular layer in contact with the metallic substrate. For higher coverages, the well-known light-induced excited spin-state trapping effect is recovered. Combining continuous light excitation with thermal cycling, we demonstrate that at low temperature light-induced thermal hysteresis is measured for the thicker films, while for sub-monolayer coverage, the light enables extension of the thermal conversion over a large temperature range. Mechanoelastic simulations underline that, due to the intermolecular interactions, opposite behaviors are observed in the different layers composing the films.