Two-dimensional (2D) honeycomb lattices beyond graphene, such as germanene, appear very promising due to their outstanding electronic properties, such as the quantum spin Hall effects. While there have been many claims of germanene monolayers up to now, no experimental evidence of a honeycomb structure has been provided up to now for these grown monolayers. Using scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and density functional theory, we have elucidated the Ge-induced (109×109)R±24.5° reconstruction on Ag(111). We demonstrate that a powerful algorithm combining SXRD with STM allows us to solve a giant surface reconstruction with more than a hundred atoms per unit cell. Its extensive unit cell indeed consists of 98 2-fold or 3-fold coordinated Ge atoms, forming a periodic arrangement of pentagons, hexagons, and heptagons, with the inclusion of six dispersed Ag atoms. By analogy, we show that the (77×77)R±19.1° reconstruction obtained by segregation of Ge through an epitaxial Ag/Ge(111) film possesses a similar structure, i.e., Ge pentagons/hexagons/heptagons with a few Ag atoms. Such an organization is more stable than that of pure Ge monolayers and can be assigned to the ground state of epitaxial germanene.

ADVERTISEMENT RETURN TO ISSUEPREVLetter to the EditorNEXTReply to the Comment on “The Ground State of Epitaxial Germanene on Ag(111)”Kai ZhangKai ZhangSorbonne Université, Centre National de la Recherche Scientifique, Institut des NanoSciences de Paris INSP, F-75005 Paris, FranceMore by Kai Zhang, Marie-Christine HanfMarie-Christine HanfUniversité de Haute Alsace, CNRS, IS2M UMR7361, F-68100 Mulhouse, FranceUniversité de Strasbourg, 67081 Strasbourg, FranceMore by Marie-Christine Hanfhttps://orcid.org/0000-0003-1820-504X, Romain BernardRomain BernardSorbonne Université, Centre National de la Recherche Scientifique, Institut des NanoSciences de Paris INSP, F-75005 Paris, FranceMore by Romain Bernard, Yves BorenszteinYves BorenszteinSorbonne Université, Centre National de la Recherche Scientifique, Institut des NanoSciences de Paris INSP, F-75005 Paris, FranceMore by Yves Borensztein, Hervé CruguelHervé CruguelSorbonne Université, Centre National de la Recherche Scientifique, Institut des NanoSciences de Paris INSP, F-75005 Paris, FranceMore by Hervé Cruguel, Andrea RestaAndrea RestaSynchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, FranceMore by Andrea Restahttps://orcid.org/0000-0002-0614-1192, Yves GarreauYves GarreauSynchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, FranceUniversité Paris Cité, Laboratoire Matériaux et Phénomènes Quantiques, CNRS, F-75013 Paris, FranceMore by Yves Garreau, Alina VladAlina VladSynchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, FranceMore by Alina Vlad, Alessandro CoatiAlessandro CoatiSynchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, FranceMore by Alessandro Coati, Davide SciaccaDavide SciaccaUniv. Lille, CNRS, Centrale Lille, Univ. Polytechnique Hauts-de-France Junia-ISEN, UMR 8520 - IEMN, F-59000 Lille, FranceMore by Davide Sciacca, Bruno GrandidierBruno GrandidierUniv. Lille, CNRS, Centrale Lille, Univ. Polytechnique Hauts-de-France Junia-ISEN, UMR 8520 - IEMN, F-59000 Lille, FranceMore by Bruno Grandidierhttps://orcid.org/0000-0001-6131-7309, Mickael DerivazMickael DerivazUniversité de Haute Alsace, CNRS, IS2M UMR7361, F-68100 Mulhouse, FranceUniversité de Strasbourg, 67081 Strasbourg, FranceMore by Mickael Derivaz, Carmelo PirriCarmelo PirriUniversité de Haute Alsace, CNRS, IS2M UMR7361, F-68100 Mulhouse, FranceUniversité de Strasbourg, 67081 Strasbourg, FranceMore by Carmelo Pirrihttps://orcid.org/0000-0002-3629-1044, Philippe SonnetPhilippe SonnetUniversité de Haute Alsace, CNRS, IS2M UMR7361, F-68100 Mulhouse, FranceUniversité de Strasbourg, 67081 Strasbourg, FranceMore by Philippe Sonnethttps://orcid.org/0000-0001-5891-5500, Régis StephanRégis StephanUniversité de Haute Alsace, CNRS, IS2M UMR7361, F-68100 Mulhouse, FranceUniversité de Strasbourg, 67081 Strasbourg, FranceMore by Régis Stephanhttps://orcid.org/0000-0003-0345-1387, and Geoffroy Prévot*Geoffroy PrévotSorbonne Université, Centre National de la Recherche Scientifique, Institut des NanoSciences de Paris INSP, F-75005 Paris, France*Email for G.P.: [email protected]More by Geoffroy PrévotCite this: ACS Nano 2023, 17, 22, 22149–22151Publication Date (Web):November 28, 2023Publication History Received22 September 2023Accepted2 November 2023Revised27 October 2023Published online28 November 2023Published inissue 28 November 2023https://doi.org/10.1021/acsnano.3c09178Copyright © Published 2023 by American Chemical SocietyRequest reuse permissions This publication is free to access through this site. Learn MoreArticle Views-Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-AlertscloseSUBJECTS:Alloys,Density functional theory,Epitaxy,Layers,Scanning tunneling microscopy Get e-Alerts

Several studies have reported the possible growth of germanene on various metal surfaces. However, the exact structure of the layers formed upon evaporation at or above room temperature remains controversial. In particular, the layers obtained after Ge deposition on Al(111) have been either considered as honeycomb organization of Ge atoms on top of a nonreconstructed substrate, or as alloyed layers. Using quantitative measurements by surface x-ray diffraction compared to density functional theory calculations, we show that the (root 7 x root 7)R +/- 19.1 degrees reconstruction obtained after room temperature deposition is a mixed Ge-Al honeycomb layer, on top of an Al(111) plane, and not a pure germanene layer.

Germanane is a two-dimensional material consisting of stacks of atomically thin germanium sheets. It’s easy and low-cost synthesis holds promise for the development of atomic-scale devices. However, to become an electronic-grade material, high-quality layered crystals with good chemical purity and stability are needed. To this end, we studied the electrical transport of annealed methyl-terminated germanane microcrystallites in both high vacuum and ultrahigh vacuum. Scanning electron microscopy of crystallites revealed two types of behavior which arise from the difference in the crystallite chemistry. While some crystallites are hydrated and oxidized, preventing the formation of good electrical contact, the four-point resistance of oxygen-free crystallites was measured with multiple tips scanning tunneling microscopy, yielding a bulk transport with resistivity smaller than 1 Ω·cm. When normalized by the crystallite thickness, the resistance compares well with the resistance of hydrogen-passivated germanane flakes found in the literature. Along with the high purity of the crystallites, a thermal stability of the resistance at 280 °C makes methyl-terminated germanane suitable for complementary metal oxide semiconductor back-end-of-line processes.

Two-dimensional (2D) honeycomb lattices beyond graphene, such as germanene, promise new physical properties such as quantum spin Hall effect. While there have been many claims of growth of germanene, the lack of precise structural characterization of the epitaxial layers synthesized hinders further research. The striped layer formed by Ge deposition on Ag(111) has been recently ascribed as a stretched germanene layer. Using surface X-ray diffraction and density functional theory calculations, we demonstrate that it corresponds in fact to a Ag 2 Ge surface alloy with an atomic density 6.45% higher than the Ag(111) atomic density. The overall structure is formed by stripes associated with a face-centered cubic top-layer alignment, alternating with stripes associated with an hexagonal-close-packed top-layer alignment, in great analogy with the (22 × √ 3) Au(111) reconstruction.

Unlike silicene, for which the demonstration of its existence has been done through numerous independent studies, the possibility of growing epitaxial germanene remains highly controversial. It has been recently shown by scanning tunneling microscopy that the (3 x 3) surface reconstruction formed upon Ge deposition on Al(111) presents a honeycomb structure, and it was assigned to a pure germanene monolayer. Using quantitative measurements by surface X-ray diffraction compared to density functional theory calculations, we demonstrate that this Ge/Al(111) (3 x 3) reconstruction corresponds, in fact, to a mixed Ge--Al honeycomb layer on top of an alloyed interfacial layer. The model of a germanene monolayer on top of the Al(111) surface can be completely excluded.

Electron states in semiconductor materials can be modified by quantum confinement. Adding to semiconductor heterostructures the concept of lateral geometry offers the possibility to further tailor the electronic band structure with the creation of unique flat bands. Using block copolymer lithography, we describe the design, fabrication, and characterization of multiorbital bands in a honeycomb In0.53Ga0.47As/InP heterostructure quantum well with a lattice constant of 21 nm. Thanks to an optimized surface quality, scanning tunnelling spectroscopy reveals the existence of a strong resonance localized between the lattice sites, signature of a p-orbital flat band. Together with theoretical computations, the impact of the nanopatterning imperfections on the band structure is examined. We show that the flat band is protected against the lateral and vertical disorder, making this industry-standard system particularly attractive for the study of exotic phases of matter.

Despite the wealth of tunneling spectroscopic studies performed on silicene and germanene, the observation of a well-defined Dirac cone in these materials remains elusive. Here, we study germanene grown on Al(1 1 1) at submonolayer coverages with low temperature scanning tunneling spectroscopy. We show that the tunnelling spectra of the Al(1 1 1) surface and the germanene nanosheets are identical. They exhibit a clear metallic behaviour at the beginning of the experiments, that highlights the strong electronic coupling between the adlayer and the substrate. Over the course of the experiments, the spectra deviate from this initial behaviour, although consecutive spectra measured on the Al(1 1 1) surface and germanene nanosheets are still similar. This spectral diversity is explained by modifications of the tip apex, that arise from the erratic manipulation of the germanium adlayer. The origin of the characteristic features such as a wide band gap, coherence-like peaks or zero-bias anomalies are tentatively discussed in light of the physical properties of Ge and AlGe alloy clusters, that are likely to adsorb at the tip apex.

Halide perovskite nanocrystals hold promise for printable optoelectronic and photonic applications. Doping enhances their functionalities and is being investigated for substituting lead with environmentally friendlier elements. The most investigated dopant is Mn2+ that acts as a color center sensitized by the host excitons. The sensitization mechanism is far from understood and no comprehensive picture of the energy-transfer process has been proposed. Similarly, the role of shallow states, particularly abundant in defect tolerant materials, is still unknown. Here, we address this problem via spectroscopic studies at controlled excitation density and temperature on Mn:CsPbCl3 nanocrystals. Our results indicate a two-step process involving exciton localization in a shallow metastable state that mediates the thermally assisted sensitization of the Mn2+ emission, which is completely quenched for T < 200 K. At T <= 60 K, however, such emission surprisingly reappears, suggesting direct energy transfer from band-edge states. Electron spin resonance supports this picture, revealing the signatures of conformational rearrangements below 70 K, possibly removing the potential barrier for sensitization. Our results demystify anomalous behaviors of the exciton-to-Mn2+ energy-transfer mechanism and highlight the role of shallow defects in the photophysics of doped perovskite nanostructures.

Structure et propriétés électroniques des matériaux 2D à base de germanium Dans cette thèse, nous avons étudié les propriétés de deux matériaux 2D constitués de germanium : le germanène, l'équivalent du graphène, et un empilement multicouche de germanane à terminaison méthyle. En raison d’une structure atomique gauchie et d’un couplage spin orbite important, ces matériaux se démarquent du graphène et du graphite. Bien que très étudiés théoriquement, leurs propriétés physiques restent encore peu caractérisées. Dans le cas du germanène, l’étude de ce matériau a été réalisée en déposant du germanium sur une surface d’aluminium (111) sous ultravide. Pour des températures de croissance relativement basses, autour de 100°C, le germanène est épitaxié avec deux structures : la phase (3x3) et la phase (√7x√7). La microscopie à effet tunnel a été utilisée pour approfondir notre connaissance de ces phases. Dans un premier temps nous nous sommes intéressés aux propriétés électroniques. Des mesures spectroscopiques par microscopie à effet tunnel ont été réalisées à des températures de 77K et 5K. Elles n’ont malheureusement pas permis de conclure quant à la véritable nature du germanène en raison du fort couplage électronique de ce matériau avec la surface d’aluminium. Toutefois, au travers d’une diversité inattendue de spectres, cette analyse a révélé la faible adhésion du germanène à la surface Al(111), ce qui conduit à une contamination fréquente de l’apex de la pointe du microscope par les atomes de la surface. Parallèlement aux mesures spectroscopiques, la croissance de feuillets aux dimensions réduites a permis d’étudier la structure des bords des feuillets. Les observations par microscopie à effet tunnel montrent que ces feuillets croissent dans le plan des terrasses atomiques d’aluminium. Leurs bords présentent généralement un contraste plus clair que le reste du feuillet. Pour mieux comprendre ce changement de contraste, des calculs ab-initio basés sur la théorie de la fonctionnelle de la densité (DFT), ont été développés. Ils ont montré le rôle clé des atomes d’aluminium dans la formation des bords possédant quelque soit la structure zigzag ou « armchair ». A l’inverse du germanène mono-feuillet qui requiert une épitaxie, des cristaux de germanane peuvent être synthétisé par voie chimique, ce qui assure un découplage électronique du matériau avec son environnement. Nous avons réalisé une analyse multiphysique de tels cristaux passivés par des groupements méthyles, qui révèlent deux types de cristaux. Les plus gros, autour de 10 m de dimension latérale, sont polycristallins, recèlent des molécules intercalées entre les feuillets ou possèdent des surfaces oxydés et se chargent sous irradiation électronique en raison de la présence d’isopropanol à l’interface avec le substrat hôte. Les plus petits, identifiés comme les plus purs, sont les plus enclins à être caractérises par des mesures de transport à quatre pointes en ultravide. Ces mesures ont montré un transport de trous, qui se produit en volume. Ce résultat inattendu pour un matériau lamellaire suggère la présence de défauts et d’imperfection dans le plan des feuillets qui appellent à un meilleur contrôle de la synthèse des cristaux pour rendre possible l’étude des propriétés physiques fondamentales de ces cristaux.