Ultramicroscopy 278 (2025): 114243

https://doi.org/10.1016/j.ultramic.2025.114243

The preparation of ultra-thin PtSn-alloyed layers by molecular beam epitaxy was studied using low-energy electron microscopy (LEEM) and micro-diffraction (μ-LEED). Deposition at a sample temperature of 435 °C initially results in the formation of a Pt₃Sn/Pt(111) layer showing a (2 × 2) reconstruction. With continued Sn deposition, a Pt₂Sn/Pt(111) layer develops, showing a (√3 × √3)R30° reconstruction. An ultra-thin tin oxide was formed from the (2 × 2) surface by exposure to molecular oxygen at temperatures of 500 °C and 590 °C, respectively. LEED shows the evolution of a new surface structure, which could be identified as an incommensurate rectangular (2.3 0 1.8​ 3.6​) reconstruction with lattice parameters of a = (6.4 ± 0.1) Å  and b = (8.6 ± 0.1) Å  present in three domains rotated by 120° with respect to each other. This structure can be related to the zigzag reconstructions found for similar ultra-thin oxide systems. Contrarily, the (√3 × √3)R30° structure showed no oxide formation even after extensive exposure to molecular oxygen. The usage of atomic oxygen, however, allows for oxidation of this surface and the growth of thicker oxides on both types of overlayers. At 500 °C this process is accompanied by substantial roughening of the surface.

Applied Physics Reviews 12 (2025): 041413

https://doi.org/10.1063/5.0293660

As an exemplary member of the MXene family belonging to the class of two-dimensional materials, titanium carbide (Ti₃C₂T_x) MXene stands bright and is explored owing to its exceptional tunable properties. The full ambient oxidation of MXene in a spectrum of varying elevated temperatures toward the application of memristor devices is reported for the first time in this work. A Ti₃C₂T_x MXene free-standing film is oxidized in air from the temperature of 100 to 700 °C upon which the MXene completely transforms into the TiO₂ film yet retaining its free-standing nature in the form of MXene-derived TiO₂ films. Extensive surface, morphological, and bulk characterizations, such as x-ray photoelectron spectroscopy, transmission electron microscopy, and x-ray diffraction, confirmed the increasing Ti–O and decreasing Ti–C bond strength amid increasing oxidation. Furthermore, exceptional resistance switching properties are unveiled employing these heated MXene devices in tri-layer memristors utilizing flexible reduced graphene oxide as electrodes. The memristor device utilizing Ti₃C₂T_x MXene heated at 700 °C exhibited outstanding performance compared to the other series of devices with low switching voltage, a high OFF/ON ratio of >10², cycle-to-cycle repeatability, and exceptional endurance of over 6000 cycles. This work on MXene-derived TiO₂ free-standing films will lay open ways to obtain oxide based flexible electronic devices through easy fabrication methods along with the possible capability to mimic unmatched synaptic features.

J. Phys. Energy7 (2025): 025012

https://doi.org/10.1088/2515-7655/adbad9

The work's objective is to enhance the generation of H₂ via the thermochemical water splitting (TCWS) reaction over nanocrystalline mixed oxide Ce_1−xU_xO₂. While CeO₂ is the most active and stable known reducible oxide for the TCWS reaction, it is below par to make it practical. This has motivated many works to enhance its reduction capacity and therefore increase its activity. In this work the presence of both metal cations (Ce⁴⁺ and U⁴⁺) has allowed for the charge transfer reaction to occur (Ce⁴⁺ + U⁴⁺ ➔ Ce³⁺ + U⁵⁺) and therefore increased its capacity to generate oxygen vacancies, V_O (2 Ce³⁺ + V_O), needed for the TCWS reaction. Test reactions on the polycrystalline mixed oxides indicated that small atomic percentages of U (<10%) were found to be optimal for H₂ production (ca. 7 µmol g⁻¹) due to a considerable increase of Ce³⁺ states. Further studies of the Ce–U interaction were performed on thin epitaxial Ce_1−xU_xO₂ (111) films of about 6 nm. In situ x-ray photoelectron spectroscopy showed clear evidences of charge transfer at low U content (ca. 50% of surface/near surface Ce⁴⁺ cations were reduced in the case of Ce_0.95U_0.05O_2−δ). Moreover, it was found that while increasing the content of U decreased the charge transfer efficiency, it protected reduced Ce³⁺ from being oxidized. Our computational results using the DFT + U method gave evidence of charge transfer at 3.5 and 6.2 at.% of U. In agreement with experiments, theoretical calculations also showed that the charge transfer is sensitive to the distribution of U⁴⁺ around the Ce⁴⁺ cations, which in turn affected the creation of V_O needed for water splitting. Our results point out to the important yet often neglected effect of statistical entropy (cations distribution in the lattice), in addition to composition, in increasing the density of reduced states and consequently enhancing H₂ production from water.

Composite Interfaces (2025): 1-21

https://doi.org/10.1080/09276440.2024.2448883

This research aims to enhance fibre-matrix adhesion in bio-based fibre-reinforced polyolefins without using adhesion promoters. The primary focus is to establish a cross-linking mechanism between cellulose fibres and polyethylene by applying UV irradiation to a UV-transparent matrix and UV-absorbing fibres. The influence of UV treatment on the composite properties is evaluated by tensile, interfacial and interlaminar shear strength tests. The UV irradiation decreases the critical fragment length in single fibre fragmentation tests, indicating an improved fibre-matrix adhesion. The UV-irradiated composites’ tensile strength and Young’s modulus are found to be ~10% (for 3- and 8-minute irradiation) and ~50% (for 8-minute irradiation), respectively, higher than those of the untreated samples. Furthermore, the UV irradiation leads to an improvement in the interlaminar shear strength by 25%. The variation of the UV-irradiation time (3 min and 8 min) and the comparison of the properties of semi-finished composite sheets and composites also reveal chemical and physical changes in the regenerated cellulose fibres due to heat adsorption. The proposed mechanism of interfacial crosslinking is confirmed by FTIR spectroscopy. The results suggest an approach to overcome poor compatibility between hydrophobic polyolefin matrix and hydrophilic cellulose-based fibres, resulting in adhesive-free bio-based composites.

Carbon 231 (2025): 119600

https://doi.org/10.1016/j.carbon.2024.119600

Ruthenium is emerging as a promising candidate to replace copper in highly integrated electronics by enabling barrierless metallization in ultrathin interconnects. From this perspective, the study of graphene growth on such surface templates is of paramount importance as a platform for graphene integration in electronic devices. In particular, graphene growth on the Ru(101‾0) surface allows selective growth of different graphene orientations, one-dimensional structures, and reduced substrate interaction compared to the well-established hexagonal Ru(0001) substrate. Real-time growth observations using low-energy electron microscopy and micro-diffraction highlight the influence of substrate symmetry on graphene growth, leading to the formation of rectangular islands with distinct zigzag- or armchair-terminated edges. Bilayer formation on Ru(101‾0) occurs by nucleation of graphene nanoribbons under the monolayer. Micro-spot angle-resolved photoemission spectroscopy shows significantly less charge-transfer doping in these freestanding, zigzag-terminated bilayer graphene nanoribbons, indicating reduced graphene-substrate interaction and hence more effective decoupling as compared to graphene/Ru(0001). Our results show that the growth of graphene on non-hexagonal substrates opens new pathways for tailoring the graphene-substrate interaction at the interface, and thus the properties of graphene beyond the limits imposed by hexagonal substrates.

2D Materials 11(2) (2024): 025031

DOI: 10.1088/2053-1583/ad3134

MoS₂ and WS₂ mono- and multilayers were grown on SiO₂/Si substrates. Growth by atomic layer deposition (ALD) at fast growth rates is compared to sub-ALD, which is a slow growth rate process with only partial precursor surface coverage per cycle. A Raman spectroscopic analysis of the intensity and frequency difference of the modes reveals different stages of growth from partial to full surface layer coverage followed by layer-by-layer formation. The initial layer thickness and structural quality strongly depend on the growth rate and monolayers only form using sub-ALD. Optical activity is demonstrated by photoluminescence (PL) characterization which shows typical excitonic emission from MoS₂ and WS₂ monolayers. A chemical analysis confirming the stoichiometry of MoS₂ is performed by x-ray photoelectron spectroscopy. The surface morphology of layers grown with different growth rates is studied by atomic force microscopy. Plan-view transmission electron microscopy analysis of MoS₂ directly grown on freestanding graphene reveals the local crystalline quality of the layers, in agreement with Raman and PL results.

APL Materials 12 (2024): 011120

https://doi.org/10.1063/5.0180041

The growth of α-Ga₂O₃ and α-(In_xGa_1−x)₂O₃ on m-plane α-Al₂O₃(⁠101̄0⁠) by molecular beam epitaxy (MBE) and metal-oxide-catalyzed epitaxy (MOCATAXY) is investigated. By systematically exploring the parameter space accessed by MBE and MOCATAXY, phase-pure α-Ga₂O₃(⁠101̄0⁠) and α-(In_xGa_1−x)₂O₃(⁠101̄0⁠) thin films are realized. The presence of In on the α-Ga₂O₃ growth surface remarkably expands its growth window far into the metal-rich flux regime and to higher growth temperatures. With increasing O-to-Ga flux ratio (R_O), In incorporates into α-(In_xGa_1−x)₂O₃ up to x ≤ 0.08. Upon a critical thickness, β-(In_xGa_1−x)₂O₃ nucleates and, subsequently, heteroepitaxially grows on top of α-(In_xGa_1−x)₂O₃ facets. Metal-rich MOCATAXY growth conditions, where α-Ga₂O₃ would not conventionally stabilize, lead to single-crystalline α-Ga₂O₃ with negligible In incorporation and improved surface morphology. Higher T_TC further results in single-crystalline α-Ga₂O₃ with well-defined terraces and step edges at their surfaces. For R_O ≤ 0.53, In acts as a surfactant on the α-Ga₂O₃ growth surface by favoring step edges, while for R_O ≥ 0.8, In incorporates and leads to a-plane α-(In_xGa_1−x)₂O₃ faceting and the subsequent (⁠2̄01) β-(In_xGa_1−x)₂O₃ growth on top. Thin film analysis by scanning transmission electron microscopy reveals highly crystalline α-Ga₂O₃ layers and interfaces. We provide a phase diagram to guide the MBE and MOCATAXY growth of single-crystalline α-Ga₂O₃ on α-Al₂O₃(⁠101̄0⁠).

Advanced Engineering Materials 25 (2023): 2300942

https://doi.org/10.1002/adem.202300942

Herein, feasibility of plasmonically enhanced molecule detection via surface-enhanced Raman scattering for ceramics that are commonly used as bone ortooth replacement materials is evaluated. Open cell foams of Bioglass 45S5, thecommercial hydroxyapatite-based product Bio-Oss, and bioinert zirconia-toughened-alumina (ZTA) are coated with Au nanoparticles via colloidal depo-sition to introduce plasmonic effects. Depending on the pore size, gold-func-tionalized plasmonic porous Bioglass shows effective Raman enhancementfactor (eEF) up to 5.4
·10⁴, while depositing gold nanoparticles on Bio-Oss andporous ZTA resulted in eEF of 1.1
·10⁴ and 2.4·
10⁵ respectively. The per-formance of the plasmonic porous bioceramics under simulated biologicalconditions is examined in situ in the biological medium fetal bovine serum (FBS)and during extended incubation in mineralizing simulated bodyfluid (SBF). Mostnotably, the plasmonic porous Bioglass still delivered an eEF around 7.2
·10³ after 28 days of incubation in SBF, indicating promising stability in simulatedbiological conditions without significant difference in SBF bioactivity before andafter Au deposition. Accordingly, the plasmonically enhanced porous bioceramicsoffer the possibility for real-time and sensitive molecule detection at SBF and FBSconditions and can be further developed for sensing of specific biomarkers, forexample, in the context of osseointegration of bone replacement materials.

ACS Appl. Nano Mater. 5 (2022): 17702–17710

https://doi.org/10.1021/acsanm.2c03584

The nucleation and growth of single-layer molybdenum disulfide single-domain nanosheets is investigated by in situ low-energy electron microscopy. We study the growth of micrometer-sized flakes and the correlated flattening process of the gold surface for three different elevated temperatures. Furthermore, the influence of surface step edges on the molybdenum disulfide growth process is revealed. We show that both nanosheet and underlying terrace grow simultaneously by pushing the surface step in the expansion process. Our findings point to an optimized growth procedure allowing for step-free, single-domain, single-layer islands of several micrometers in size, which is likely transferable to other transition-metal dichalcogenides (TMDs), offering a very fine degree of control over the TMD nanosheet structure and thickness.

J. Phys. Chem. C. 126 (2022): 19101–19112

https://doi.org/10.1021/acs.jpcc.2c04174

Submonolayer coverages of V-oxide on Rh(111) condense during catalytic methanol oxidation into a pattern of macroscopic stripes or islands. Under reaction conditions, a phase separation occurs within the VO_x islands that has been studied in a pressure range of 10^–6–10^–4 mbar with photoemission electron microscopy (PEEM), low-energy electron microscopy (LEEM), microspot-low-energy electron diffraction (μLEED), and microspot-X-ray photoelectron spectroscopy (μXPS). An oxidized outer ring with a (√7 × √7)R19.1° structure coexists with an inner (12 × 12) Moiré-type boundary layer and a reduced core exhibiting a (√3 × √3)R30° Moiré type pattern. The dependence of the substructure on the reaction conditions, on V coverage, and on island size was investigated. With μXPS, the V coverages of the different phases in the VO_x islands were determined.

Open Ceramics 9 (2022): 100228

https://doi.org/10.1016/j.oceram.2022.100228

We demonstrate the feasibility of plasmonic porous ceramics which combine the optical properties of plasmonic nanoparticles with the advantages of open porous ceramics. To this end, we prepared open porous structures for surface-enhanced Raman spectroscopy (SERS) based on zirconia-toughened alumina on which we deposited silver nanoparticles. The Raman enhancement of the plasmonic structures was analyzed as a function of the amount of deposited silver nanoparticles, pore diameter and strut diameter of the ceramic structure using the probe molecule pyridine. Flat substrates of the same chemical composition and non-porous fragments of the porous structure were used for comparison. The Raman signal is found to be significantly augmented by the porous structure compared to that collected on flat substrates with similar composition. Accordingly, we propose that the plasmonic porous ceramics are well suited as 3D SERS substrates, allowing real-time Raman sensing of trace amounts of molecules.

Journal of the American Ceramic Society (2021), 105, 2702-2712

https://doi.org/10.1111/jace.18261

Mullite-type RMn₂O₅ (R = Y, rare-earth element) ceramics are of ongoing research attention because of their interesting crystal-chemical and magnetic properties. We report nuclear and magnetic structures of NdMnTiO₅ together with its spectroscopic, thermogravimetric, and magnetic properties. The polycrystalline sample is prepared by solid-state synthesis and characterized from neutron and X-ray powder diffraction data Rietveld refinements. NdMnTiO₅ crystallizes in the orthorhombic space group Pbam with metric parameter a  =  755.20(1) pm, b  =  869.91(1) pm, c  =  582.42(1) pm, and V  =  382.62(1) 10⁶ pm³. The Mn³⁺ and Ti⁴⁺ cations are observed to be located in the octahedral and pyramidal sites, respectively. The vibrational features in these polyhedral sites are characterized by Raman and Fourier transform infrared spectroscopes. The higher decomposition temperature of NdMnTiO₅, compared to other RMn₂O₅ phases, is explained in terms of the higher bond strength of Ti-O bonds than those of Mn-O bonds. Temperature-dependent DC magnetic susceptibility suggests a paramagnetic to antiferromagnetic phase transition at 43(1) K. Inverse susceptibility in the paramagnetic region above 120 K follows the Curie-Weiss law, resulting in a magnetic moment of 6.33(1) μ_B per formula unit. Neutron diffraction data collected at 7.5 K reveal that the magnetic moments of Nd³⁺ and Mn³⁺ in NdMnTiO₅ are incommensurately ordered with a propagation vector k = (0, 0.238, 0.117).

Zeitschrift für Kristallographie - Crystalline Materials 236 (2021): 283-292

https://doi.org/10.1515/zkri-2021-2046

Pb₂Sc₂Si₂O₉ and Pb₂In₂Si₂O₉, respectively, the scandium and indium containing structural analogues of the mineral kentrolite are grown by spontaneous crystallization from a PbO flux. The corresponding polycrystalline powder samples are synthesized by conventional solid-state approach. The compounds are thoroughly characterized using temperature-dependent single crystal and powder X-ray diffraction, heat capacity measurements, second harmonic generation experiments and Raman spectroscopy. At ambient conditions, both compounds crystallize in the non-centrosymmetric Pna2₁ space group and undergo phase transitions to the centrosymmetric Pbcn space group at elevated temperatures. The Pbcn into Pna2₁ phase transitions are complemented by the signals of the temperature-dependent second harmonic generation. The specific heat capacity exhibits distinct cusp, supporting the λ-type second-order phase transition. The temperature dependency of some selective Raman modes further complements the findings, showing softening and hardening of the phonons across the phase transitions.

Journal of Physical Chemistry C 125 (2021): 22539–22546

https://doi.org/10.1021/acs.jpcc.1c06106

With LEEM (low-energy electron microscopy) and micro-LEED as in situ techniques we have studied the structural transitions in the excitation of traveling interface pulses (TIPs) in bistable methanol oxidation on a bare Rh(110) and on a Rh(110) surface covered with a 0.1 monolayer of V oxide in the 10^–4 mbar range. Close to equistability, a (1×1) structure coexists with O-induced reconstructions of the “missing row” type at the interface. An oxidation pulse traveling along the interface exhibits a substructure consisting of various reconstructions of the “missing row” type; on the reduced surface, the slow development of a c(2×2) structure is accompanied by a strong loss of the (0,0)-beam intensity. The addition of 0.1 monolayer of V oxide increases structural disorder but causes no qualitative changes in the structural transitions.

Front. Phys. 9 (2021): 654845

https://doi.org/10.3389/fphy.2021.654845

The transition from single-layer to bilayer growth of molybdenum disulfide on the Au(111) surface is investigated by in situ low-energy electron and photoemission microscopy. By mapping the film morphology with nanometer resolution, we show that a MoS₂ bilayer forms at the boundaries of single-layer single-domain MoS₂ islands and next to merging islands whereas bilayer nucleation at the island centers is found to be suppressed, which may be related to the usage of dimethyl disulfide as sulfur precursor in the growth process. This approach, which may open up the possibility of growing continuous films over large areas while delaying bilayer formation, is likely transferable to other transition metal dichalcogenide model systems.

Nature Scientific Reports 10 (2020): 22374

https://doi.org/10.1038/s41598-020-78584-9

Vanadium dioxide (VO₂) features a pronounced, thermally-driven metal-to-insulator transition at 340 K. Employing epitaxial stress on rutile TiO2(001) substrates, the transition can be tuned to occur close to room temperature. Striving for applications in oxide-electronic devices, the lateral homogeneity of such samples must be considered as an important prerequisite for efforts towards miniaturization. Moreover, the preparation of smooth surfaces is crucial for vertically stacked devices and, hence, the design of functional interfaces. Here, the surface morphology of VO2/TiO2(001) films was analyzed by low-energy electron microscopy and diffraction as well as scanning probe microscopy. The formation of large terraces could be achieved under temperature-induced annealing, but also the occurrence of facets was observed and characterized. Further, we report on quasi-periodic arrangements of crack defects which evolve due to thermal stress under cooling. While these might impair some applicational endeavours, they may also present crystallographically well-oriented nano-templates of bulk-like properties for advanced approaches.

Inorg. Chem 59 (2020): 18214–18224

https://doi.org/10.1021/acs.inorgchem.0c02684

We report a detailed structural, spectroscopic, and thermogravimetric investigation of a new series of mixed-alkali rare-earth orthoborates KLi₂RE(BO₃)₂ (RE = Dy, Ho, Er, Tm, Yb, and Y). Single crystals were directly prepared by a flux method as well as mechanically separated from the polycrystalline powder obtained from the conventional solid-state reactions. All KLi₂RE(BO₃)₂ members are isotypic and crystallize in the space group P2₁/n. The novel structure type is comprised of [RE₂(BO₃)₄O₄]^14– anionic clusters where the edge-sharing REO₇ pentagonal bipyramids are connected by BO₃ groups and both K⁺ and Li⁺ cations are located at the interstitial voids of the 3D network. The metric parameters and crystal structural features obtained from the single-crystal data are in excellent agreement with those refined from the powder data. The change of the lattice parameters and unit cell volumes can be explained in terms of the lanthanide contraction effect. A comparison between KLi₂RE(BO₃)₂ and other rare-earth borates with similar chemical compositions indicates that the sum of the ionic radii of the alkali-metal cations governs the symmetry of the crystals. Diffuse reflectance UV–vis spectra display the characteristic absorption behaviors of the RE³⁺ cations and the fundamental absorption edge. Both the Tauc’s and derivation of absorption spectrum fitting (DASF) methods were used to identify the magnitude and type of bandgap, respectively, which are compared with those obtained from density functional theory (DFT) calculations. The calculated phonon density of states and the vibrational frequency at the gamma point help explain the Fourier transform infrared and Raman spectra of KLi₂RE(BO₃)₂. The incongruent melting behavior and the thermal stability of each member of the KLi₂RE(BO₃)₂ series were also studied by thermogravimetric analyses.

Surface Science 694 (2020): 121561

https://doi.org/10.1016/j.susc.2019.121561

The adsorption of S on Si(111)- 7  ×  7 has been investigated for different preparation schemes and parameters. S was supplied from an electrochemical Ag₂S cell. For room temperature adsorption and subsequent annealing, no ordered S induced reconstruction can be observed with spot profile analysis low-energy electron diffraction (SPALEED). S deposition at temperatures above about 400 ^∘C, however, leads to a well-ordered reconstruction. Judging from the LEED pattern, the same reconstruction was already observed by Metzner et al. [Surf. Sci. 377–379 (1997) 71–74] who identified it as 4  ×  4 reconstruction. The upper temperature limit for the preparation of this superstructure depends on S flux, which is needed to compensate for desorption. Prolonged S exposure leads to surface roughening, as observed with SPALEED and scanning tunneling microscopy (STM), pointing to surface etching by S. From our SPALEED data, we can conclude that the observed reconstruction is not a 4  ×  4 reconstruction, but a superstructure with a rectangular unit cell that exists in three rotational domains, as confirmed by STM. Different structural trial models have been assessed with density functional theory. Among these model structures, a configuration with dimers adsorbed on bridging sites, with a S coverage of 1 monolayer, is most likely, since it is energetically favorable and is in agreement with all experimental results.

Nano Letters (2019) 19, 6554–6563

https://doi.org/10.1021/acs.nanolett.9b02798

As a key player in blood coagulation and tissue repair, fibrinogen has gained increasing attention to develop nanofibrous biomaterial scaffolds for wound healing. Current techniques to prepare protein nanofibers, like electrospinning or extrusion, are known to induce lasting changes in the protein conformation. Often, such secondary changes are associated with amyloid transitions, which can evoke unwanted disease mechanisms. Starting from our recently introduced technique to self-assemble fibrinogen scaffolds in physiological salt buffers, we here investigated the morphology and secondary structure of our novel fibrinogen nanofibers. Aiming at optimum self-assembly conditions for wound healing scaffolds, we studied the influence of fibrinogen concentration and pH on the protein conformation. Using circular dichroism and Fourier-transform infrared spectroscopy, we observed partial transitions from α-helical structures to β-strands upon fiber formation. Interestingly, a staining with thioflavin T revealed that this conformational transition was not associated with any amyloid formation. Toward novel scaffolds for wound healing, which are stable in aqueous environment, we also introduced cross-linking of fibrinogen scaffolds in formaldehyde vapor. This treatment allowed us to maintain the nanofibrous morphology while the conformation of fibrinogen nanofibers was redeveloped toward a more native state after rehydration. Altogether, self-assembled fibrinogen scaffolds are excellent candidates for novel wound healing systems since their multiscale structures can be well controlled without inducing any pathogenic amyloid transitions.

Physical Chemistry Chemical Physics (2017) 19, 3480-3485

https://doi.org/10.1039/C6CP06853G

The growth, morphology, structure, and stoichiometry of ultrathin praseodymium oxide layers on Ru(0001) were studied using low-energy electron microscopy and diffraction, photoemission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. At a growth temperature of 760 °C, the oxide is shown to form hexagonally close-packed (A-type) Pr₂O₃(0001) islands that are up to 3 nm high. Depending on the local substrate step density, the islands either adopt a triangular shape on sufficiently large terraces or acquire a trapezoidal shape with the long base aligned along the substrate steps.

Journal of Physics: Condensed Matter (2016) 28, 475003

https://doi.org/10.1088/0953-8984/28/47/475003

The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.