{"id":{"repo_id":"tu-berlin","oai_identifier":"oai:depositonce.tu-berlin.de:11303/21299"},"canonical_url":"https://search.dev.ndltd.org/etd/tu-berlin/oai:depositonce.tu-berlin.de:11303/21299","repository":{"repo_id":"tu-berlin","name":"Technische Universität Berlin","base_url":"https://api-depositonce.tu-berlin.de/server/oai/request"},"display":{"title":"Characterization of bio-hybrid interfaces under ambient conditions","abstract":"The topography, morphology, and chemistry of the surface of HVPE-grown n-GaN(0001) layers on c-sapphire/n-GaN(0001) samples were explored under various conditions before and after sputtering with 1 keV nitrogen N2 ions, followed by the functionalization with deionized water (H2O) and neutral L-cysteine (C3H7NO2S) molecules. Atomic force microscopy (AFM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) in ultra-high vacuum (UHV) and near-ambient-pressure (NAP) conditions were used for this investigation. Monochromatic X-ray 𝐴𝐴𝐴𝐴 𝐾𝐾𝐾𝐾 radi ation (ℎ𝜋𝜋 = 1486.7𝑒𝑒𝑉𝑉) was the exc itat ion source for the XPS experiments. The results obtained yielded information on the modifications occurring at the surface of the n-GaN(0001) layers upon N2+-sputtering and on the contact formation and interfacial chemical reactivity after exposure to H2O and L-cysteine, i.e., on the ensuing interactions and bonding mechanisms between H2O and L-cysteine molecules and the N2+-sputtered n-GaN(0001) surface. This provided a clue on the suitability, stability, and functionality of GaN-OH and GaN-SH groups, the primary links between the surface and inorganic, organic and biomolecules, and hence, on the potentiality of using GaN-based surfaces for electronic devices and biosensors. Experimental UHV-XPS spectra of C1s and O1s core-level (CL) transitions of the as-grown n-GaN(0001) surface revealed adsorbed adventitious carbon (C) and oxygen (O2) species, besides traces of Ga2O3 and Ga2Ox oxides. The surface AFM images and LEED patterns revealed that N2+-ion sputtering resulted in minimal surface damage and roughness, in addition to largely reducing the amount of adsorbed carbon species. The NAP-XPS spectra of the Ga 2p3/2 and Ga 3d5/2 photoemission lines of the N2+-sputtered n-GaN(0001)/H2O interface showed that the H2O molecules adsorbed on the n-GaN(0001) surface likely dissociated into a hydroxyl ion (OH−) and a proton (H+), which tend to combine with surface gallium (Ga+) dangling bonds and unbonded nitrogen (N−) ions, respectively. The N2+-ion sputtering and H2O exposure modified the valence band (VB) structure of the as-grown n-GaN(0001) surface. This is shown by the respective UHV and NAP-XPS VB photoemission spectra, which displayed VB peaks related to the hybridized orbital states of apolar surface dominated by Ga, besides revealing a reduction in the band bending at the N_2^+-sputtered n-GaN(0001) surface. Functionalization of the N_2^+-sputtered n-GaN(0001) surface with H2O molecules induced a noticeable modification to its chemistry and band bending (BB), disclosed from the measured NAP-XPS O 1s and Ga 3d lines, and from the NAP-XPS VB spectra of the ensuing N_2^+-sputtered n-GaN(0001)/H2O interface. The formation of Ga-OH and N-H bonds, and also Ga-O bonds that become prominent at high temperatures, are inferred from the NAP-XPS peaks of the existing surface constituents. Such a compensation of dangling bonds and surface states upon increasing the amount of H2O exposure typified itself as a continuous decrease in the surface BB voltage V_bb up to a pressure p=0.1 mbar, above which V_bb leveled off at a constant value, depending on the sample temperature T. The surface BB voltage V_bb was estimated by two methodologies. One approach used the energy difference (E_FS-E_VS) between the surface valence-band edge (E_VS) and the Fermi level (E_FS), the VBM energy, by the linear extrapolation method. The other approach monitored the measured peak energy of the Ga 3dS line of the N_2^+-sputtered n-GaN(0001) surface and the N_2^+-sputtered n-GaN(0001)/H2O interface viz., (E_FS-E_Ga3dS), combined with 17.5 eV, the energy difference between VB edge and Ga 3dB CL of bulk GaN. The two approaches incorporated the energy differences between the conduction band (CB) edge (E_CB) and the Fermi energy E_FB of bulk GaN (E_CB-E_FB) and between E_CB and E_VB, the band gap energy E_g, both of which are temperature dependent. This partially explains the observed decrease in V_bb with increasing T. Due to the difficulty of finding a proper linear extrapolation for the VB spectra of the N_2^+-sputtered n-GaN(0001)/H2O interface, mainly at high H2O pressure, V_bb was estimated from the binding energy shifts of (E_FS-E_Ga3dS ), combined with (E_VB-E_Ga3dB )=17.5eV, and (E_FS-E_VS ), the VBM energy. At a constant T, the decrease in V_bb at the N_2^+-sputtered n-GaN(0001)/H2O interface with pressure p below 0.1 mbar is ascribed to the compensation of surface dangling bonds by dissociative H2O molecules. The constancy of V_bb for p>0.1 mbar is understood in terms of a saturation coverage of the n-GaN(0001) surface with H2O molecules. The reduction of p>0.1 V_bb with increasing T can be partly accounted for by the decrease in E_g (T)=(E_CB-E_VB ) and (E_CB-E_FB (T)) with T, though such a decrease cannot solely clarify the entire trend of V_bb at high T. The N_2^+-sputtered n-GaN(0001) surface permits notable bonding of the sulfur-containing amino-acid L-cysteine (L-CySH) molecules, as concluded from the respective UHV-XPS spectra. Deposition of L-CySH on the n-GaN(0001) surface was made via sublimation of L-CySH powder at 105 oC for ≤ 40 min. The UHV-XPS spectra of the N_2^+-sputtered GaN(0001)/L-cysteine interface were taken at room temperature and after annealing at various temperatures (T_A=75-150℃) for annealing times (t_A) of 15, 30, and 60 minutes. The UHV-XPS spectra of the N_2^+-sputtered n-GaN(0001)/L-cysteine interface showed that the thiol head group (SH group) of the L-cysteine molecule formed strong chemical bonding with the surface metallic Ga. Such behavior was noted from the overlapped Ga 3s and S 2p XPS photoemission lines, the intensity of which were enhanced with more deposited L-cysteine layers, a feature that is characteristic for thiolate species. The bonding of carboxyl (COOH) and amino (NH2) groups to the surface cannot be entirely excluded, as was noted from the respective C 1s, O 1s, and N 1s XPS spectra. The strength and appearance of the doublet S 2p photoemission lines diminished as T_A increased, suggesting that most but not all L-cysteine layers re-evaporated from the functionalized surface, with the SH-group of the lowermost L-cysteine layer being still chemically bonded to the Ga atoms at the surface.","abstract_html":"The topography, morphology, and chemistry of the surface of HVPE-grown n-GaN(0001) layers on c-sapphire/n-GaN(0001) samples were explored under various conditions before and after sputtering with 1 keV nitrogen N2 ions, followed by the functionalization with deionized water (H2O) and neutral L-cysteine (C3H7NO2S) molecules. Atomic force microscopy (AFM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) in ultra-high vacuum (UHV) and near-ambient-pressure (NAP) conditions were used for this investigation. Monochromatic X-ray 𝐴𝐴𝐴𝐴 𝐾𝐾𝐾𝐾 radi ation (ℎ𝜋𝜋 = 1486.7𝑒𝑒𝑉𝑉) was the exc itat ion source for the XPS experiments. The results obtained yielded information on the modifications occurring at the surface of the n-GaN(0001) layers upon N2+-sputtering and on the contact formation and interfacial chemical reactivity after exposure to H2O and L-cysteine, i.e., on the ensuing interactions and bonding mechanisms between H2O and L-cysteine molecules and the N2+-sputtered n-GaN(0001) surface. This provided a clue on the suitability, stability, and functionality of GaN-OH and GaN-SH groups, the primary links between the surface and inorganic, organic and biomolecules, and hence, on the potentiality of using GaN-based surfaces for electronic devices and biosensors. Experimental UHV-XPS spectra of C1s and O1s core-level (CL) transitions of the as-grown n-GaN(0001) surface revealed adsorbed adventitious carbon (C) and oxygen (O2) species, besides traces of Ga2O3 and Ga2Ox oxides. The surface AFM images and LEED patterns revealed that N2+-ion sputtering resulted in minimal surface damage and roughness, in addition to largely reducing the amount of adsorbed carbon species. The NAP-XPS spectra of the Ga 2p3/2 and Ga 3d5/2 photoemission lines of the N2+-sputtered n-GaN(0001)/H2O interface showed that the H2O molecules adsorbed on the n-GaN(0001) surface likely dissociated into a hydroxyl ion (OH−) and a proton (H+), which tend to combine with surface gallium (Ga+) dangling bonds and unbonded nitrogen (N−) ions, respectively. The N2+-ion sputtering and H2O exposure modified the valence band (VB) structure of the as-grown n-GaN(0001) surface. This is shown by the respective UHV and NAP-XPS VB photoemission spectra, which displayed VB peaks related to the hybridized orbital states of apolar surface dominated by Ga, besides revealing a reduction in the band bending at the N_2^+-sputtered n-GaN(0001) surface. Functionalization of the N_2^+-sputtered n-GaN(0001) surface with H2O molecules induced a noticeable modification to its chemistry and band bending (BB), disclosed from the measured NAP-XPS O 1s and Ga 3d lines, and from the NAP-XPS VB spectra of the ensuing N_2^+-sputtered n-GaN(0001)/H2O interface. The formation of Ga-OH and N-H bonds, and also Ga-O bonds that become prominent at high temperatures, are inferred from the NAP-XPS peaks of the existing surface constituents. Such a compensation of dangling bonds and surface states upon increasing the amount of H2O exposure typified itself as a continuous decrease in the surface BB voltage V_bb up to a pressure p=0.1 mbar, above which V_bb leveled off at a constant value, depending on the sample temperature T. The surface BB voltage V_bb was estimated by two methodologies. One approach used the energy difference (E_FS-E_VS) between the surface valence-band edge (E_VS) and the Fermi level (E_FS), the VBM energy, by the linear extrapolation method. The other approach monitored the measured peak energy of the Ga 3dS line of the N_2^+-sputtered n-GaN(0001) surface and the N_2^+-sputtered n-GaN(0001)/H2O interface viz., (E_FS-E_Ga3dS), combined with 17.5 eV, the energy difference between VB edge and Ga 3dB CL of bulk GaN. The two approaches incorporated the energy differences between the conduction band (CB) edge (E_CB) and the Fermi energy E_FB of bulk GaN (E_CB-E_FB) and between E_CB and E_VB, the band gap energy E_g, both of which are temperature dependent. This partially explains the observed decrease in V_bb with increasing T. Due to the difficulty of finding a proper linear extrapolation for the VB spectra of the N_2^+-sputtered n-GaN(0001)/H2O interface, mainly at high H2O pressure, V_bb was estimated from the binding energy shifts of (E_FS-E_Ga3dS ), combined with (E_VB-E_Ga3dB )=17.5eV, and (E_FS-E_VS ), the VBM energy. At a constant T, the decrease in V_bb at the N_2^+-sputtered n-GaN(0001)/H2O interface with pressure p below 0.1 mbar is ascribed to the compensation of surface dangling bonds by dissociative H2O molecules. The constancy of V_bb for p>0.1 mbar is understood in terms of a saturation coverage of the n-GaN(0001) surface with H2O molecules. The reduction of p>0.1 V_bb with increasing T can be partly accounted for by the decrease in E_g (T)=(E_CB-E_VB ) and (E_CB-E_FB (T)) with T, though such a decrease cannot solely clarify the entire trend of V_bb at high T. The N_2^+-sputtered n-GaN(0001) surface permits notable bonding of the sulfur-containing amino-acid L-cysteine (L-CySH) molecules, as concluded from the respective UHV-XPS spectra. Deposition of L-CySH on the n-GaN(0001) surface was made via sublimation of L-CySH powder at 105 oC for ≤ 40 min. The UHV-XPS spectra of the N_2^+-sputtered GaN(0001)/L-cysteine interface were taken at room temperature and after annealing at various temperatures (T_A=75-150℃) for annealing times (t_A) of 15, 30, and 60 minutes. The UHV-XPS spectra of the N_2^+-sputtered n-GaN(0001)/L-cysteine interface showed that the thiol head group (SH group) of the L-cysteine molecule formed strong chemical bonding with the surface metallic Ga. Such behavior was noted from the overlapped Ga 3s and S 2p XPS photoemission lines, the intensity of which were enhanced with more deposited L-cysteine layers, a feature that is characteristic for thiolate species. The bonding of carboxyl (COOH) and amino (NH2) groups to the surface cannot be entirely excluded, as was noted from the respective C 1s, O 1s, and N 1s XPS spectra. The strength and appearance of the doublet S 2p photoemission lines diminished as T_A increased, suggesting that most but not all L-cysteine layers re-evaporated from the functionalized surface, with the SH-group of the lowermost L-cysteine layer being still chemically bonded to the Ga atoms at the surface.","abstract_has_math":false,"creators":["Ahmad, Mais"],"institution":null,"degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Esser, Norbert"],"committee_chairs":[],"committee_members":[],"year":2024,"date_issued":"2024","date_published":"2024","updated_at":"2026-07-27T21:28:52Z","subjects":[],"languages":["en"],"rights":[],"rights_urls":["https://creativecommons.org/licenses/by/4.0/"],"identifier_entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://doi.org/10.14279/depositonce-20099"],"render_values":[{"text":"https://doi.org/10.14279/depositonce-20099","href":"https://doi.org/10.14279/depositonce-20099","code":true}]}]},"links":{"outbound_url":"https://depositonce.tu-berlin.de/handle/11303/21299","outbound_label":"Repository record","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Esser, Norbert"]},{"key":"dc:creator","label":"Author","values":["Ahmad, Mais"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.accessioned","label":"Dc Date Accessioned","values":["2024-04-10T10:44:52Z"]},{"key":"dc:date.available","label":"Dc Date Available","values":["2024-04-10T10:44:52Z"]},{"key":"dc:date.issued","label":"Date","values":["2024"]},{"key":"dc:type","label":"Dc Type","values":["Doctoral Thesis"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language.iso","label":"Language (ISO)","values":["en"]},{"key":"dc:rights.uri","label":"Rights URI","values":["https://creativecommons.org/licenses/by/4.0/"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://depositonce.tu-berlin.de/handle/11303/21299","https://doi.org/10.14279/depositonce-20099"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["The topography, morphology, and chemistry of the surface of HVPE-grown n-GaN(0001) layers on c-sapphire/n-GaN(0001) samples were explored under various conditions before and after sputtering with 1 keV nitrogen N2 ions, followed by the functionalization with deionized water (H2O) and neutral L-cysteine (C3H7NO2S) molecules. Atomic force microscopy (AFM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) in ultra-high vacuum (UHV) and near-ambient-pressure (NAP) conditions were used for this investigation. Monochromatic X-ray 𝐴𝐴𝐴𝐴 𝐾𝐾𝐾𝐾 radi ation (ℎ𝜋𝜋 = 1486.7𝑒𝑒𝑉𝑉) was the exc itat ion source for the XPS experiments. The results obtained yielded information on the modifications occurring at the surface of the n-GaN(0001) layers upon N2+-sputtering and on the contact formation and interfacial chemical reactivity after exposure to H2O and L-cysteine, i.e., on the ensuing interactions and bonding mechanisms between H2O and L-cysteine molecules and the N2+-sputtered n-GaN(0001) surface. This provided a clue on the suitability, stability, and functionality of GaN-OH and GaN-SH groups, the primary links between the surface and inorganic, organic and biomolecules, and hence, on the potentiality of using GaN-based surfaces for electronic devices and biosensors. Experimental UHV-XPS spectra of C1s and O1s core-level (CL) transitions of the as-grown n-GaN(0001) surface revealed adsorbed adventitious carbon (C) and oxygen (O2) species, besides traces of Ga2O3 and Ga2Ox oxides. The surface AFM images and LEED patterns revealed that N2+-ion sputtering resulted in minimal surface damage and roughness, in addition to largely reducing the amount of adsorbed carbon species. The NAP-XPS spectra of the Ga 2p3/2 and Ga 3d5/2 photoemission lines of the N2+-sputtered n-GaN(0001)/H2O interface showed that the H2O molecules adsorbed on the n-GaN(0001) surface likely dissociated into a hydroxyl ion (OH−) and a proton (H+), which tend to combine with surface gallium (Ga+) dangling bonds and unbonded nitrogen (N−) ions, respectively. The N2+-ion sputtering and H2O exposure modified the valence band (VB) structure of the as-grown n-GaN(0001) surface. This is shown by the respective UHV and NAP-XPS VB photoemission spectra, which displayed VB peaks related to the hybridized orbital states of apolar surface dominated by Ga, besides revealing a reduction in the band bending at the N_2^+-sputtered n-GaN(0001) surface. Functionalization of the N_2^+-sputtered n-GaN(0001) surface with H2O molecules induced a noticeable modification to its chemistry and band bending (BB), disclosed from the measured NAP-XPS O 1s and Ga 3d lines, and from the NAP-XPS VB spectra of the ensuing N_2^+-sputtered n-GaN(0001)/H2O interface. The formation of Ga-OH and N-H bonds, and also Ga-O bonds that become prominent at high temperatures, are inferred from the NAP-XPS peaks of the existing surface constituents. Such a compensation of dangling bonds and surface states upon increasing the amount of H2O exposure typified itself as a continuous decrease in the surface BB voltage V_bb up to a pressure p=0.1 mbar, above which V_bb leveled off at a constant value, depending on the sample temperature T. The surface BB voltage V_bb was estimated by two methodologies. One approach used the energy difference (E_FS-E_VS) between the surface valence-band edge (E_VS) and the Fermi level (E_FS), the VBM energy, by the linear extrapolation method. The other approach monitored the measured peak energy of the Ga 3dS line of the N_2^+-sputtered n-GaN(0001) surface and the N_2^+-sputtered n-GaN(0001)/H2O interface viz., (E_FS-E_Ga3dS), combined with 17.5 eV, the energy difference between VB edge and Ga 3dB CL of bulk GaN. The two approaches incorporated the energy differences between the conduction band (CB) edge (E_CB) and the Fermi energy E_FB of bulk GaN (E_CB-E_FB) and between E_CB and E_VB, the band gap energy E_g, both of which are temperature dependent. This partially explains the observed decrease in V_bb with increasing T. Due to the difficulty of finding a proper linear extrapolation for the VB spectra of the N_2^+-sputtered n-GaN(0001)/H2O interface, mainly at high H2O pressure, V_bb was estimated from the binding energy shifts of (E_FS-E_Ga3dS ), combined with (E_VB-E_Ga3dB )=17.5eV, and (E_FS-E_VS ), the VBM energy. At a constant T, the decrease in V_bb at the N_2^+-sputtered n-GaN(0001)/H2O interface with pressure p below 0.1 mbar is ascribed to the compensation of surface dangling bonds by dissociative H2O molecules. The constancy of V_bb for p>0.1 mbar is understood in terms of a saturation coverage of the n-GaN(0001) surface with H2O molecules. The reduction of p>0.1 V_bb with increasing T can be partly accounted for by the decrease in E_g (T)=(E_CB-E_VB ) and (E_CB-E_FB (T)) with T, though such a decrease cannot solely clarify the entire trend of V_bb at high T. The N_2^+-sputtered n-GaN(0001) surface permits notable bonding of the sulfur-containing amino-acid L-cysteine (L-CySH) molecules, as concluded from the respective UHV-XPS spectra. Deposition of L-CySH on the n-GaN(0001) surface was made via sublimation of L-CySH powder at 105 oC for ≤ 40 min. The UHV-XPS spectra of the N_2^+-sputtered GaN(0001)/L-cysteine interface were taken at room temperature and after annealing at various temperatures (T_A=75-150℃) for annealing times (t_A) of 15, 30, and 60 minutes. The UHV-XPS spectra of the N_2^+-sputtered n-GaN(0001)/L-cysteine interface showed that the thiol head group (SH group) of the L-cysteine molecule formed strong chemical bonding with the surface metallic Ga. Such behavior was noted from the overlapped Ga 3s and S 2p XPS photoemission lines, the intensity of which were enhanced with more deposited L-cysteine layers, a feature that is characteristic for thiolate species. The bonding of carboxyl (COOH) and amino (NH2) groups to the surface cannot be entirely excluded, as was noted from the respective C 1s, O 1s, and N 1s XPS spectra. The strength and appearance of the doublet S 2p photoemission lines diminished as T_A increased, suggesting that most but not all L-cysteine layers re-evaporated from the functionalized surface, with the SH-group of the lowermost L-cysteine layer being still chemically bonded to the Ga atoms at the surface.","Die Topographie, Morphologie und Chemie der Oberfläche von HVPE-gewachsenen n-GaN(0001) Schichten auf c-Saphir/n-GaN(0001) Proben wurden vor und nach dem Sputtern mit 1 keV Stickstoffionen (N_2^+) und der Funktionalisierung mit entionisiertem Wasser (H2O) und neutralen L-Cystein Molekülen (C3H7NO2S) unter verschiedenen Bedingungen untersucht. Für diese Untersuchung wurden die Rasterkraftmikroskopie (AFM), die Niederenergieelektronenbeugung (LEED) und die Röntgenphotoelektronenspektroskopie (XPS) im Ultrahochvakuum (UHV) und in einer niedrig-Druck Atmosphäre (Near-Ambient-Pressure, NAP) angewendet. Die monochromatische Al-Kα Röntgenstrahlung (hν=1486.7eV) wurde bei den XPS Experimenten als Anregungsquelle eingesetzt. Die Ergebnisse lieferten Information über die durch das Sputtern verursachten Modifikationen an der Oberfläche der n-GaN(0001) Schichten, über die Kontaktbildung und die chemische Reaktivität an der Grenzfläche nach Einwirkung von H2O und L-Cystein, sowie zu die daraus resultierenden Wechselwirkungen und Bindungsmechanismen zwischen H2O und L-Cystein Molekülen und der gesputterten n-GaN(0001) Oberfläche. Dies lieferte einen Hinweis für die Eignung, Stabilität und Funktionalität von GaN-OH und GaN-SH Gruppen, den primären Verbindungen zwischen der Oberfläche und anorganischen organischen und Biomolekülen, und daher auch die Möglichkeit, GaN-basierte Oberflächen für elektronische Geräte und Biosensoren zu verwenden. Experimentelle UHV-XPS Spektren der Kernschalenübergänge C 1s und O 1s der unbehandelten n-GaN(0001) Oberfläche zeigten adsorbierte unspezifische Kohlenstoff- (C) und Sauerstoffspezies (O2) sowie Spuren von Ga2O3 und Ga2Ox Oxiden. Die AFM Bilder und LEED Muster der Oberfläche zeigten, dass das Sputtern mit N_2^+ Ionen nur minimale Oberflächenschäden und Rauheiten erzeugte, und zusätzlich die Anzahl der adsorbierten Kohlenstoffspezies deutlich verringerte. Die NAP-XPS Spektren der Ga 2p3/2 und Ga 3d5/2 Photoemissionslinien der gesputterten n-GaN(0001)/H2O Grenzfläche zeigten, dass auf der n-GaN(0001) Oberfläche adsorbierte H2O Moleküle wahrscheinlich in ein Hydroxyl Ion (OH-) und Proton (H+) dissoziierten, Ionen, die dazu neigen, sich mit ungesättigten Gallium (Ga^+) Bindungen bzw. ungebundenen Stickstoff (N^-) Ionen an der Oberfläche zu verbinden. Das Sputtern mit N_2^+ Ionen und die Abdeckung der Oberfläche durch H2O veränderten die Valenzbandstruktur (VB) der n-GaN(0001) Oberfläche, wie sie aus den jeweiligen UHV- und NAP-XPS VB Photoemissionsspektren hervorgeht. Die VB Peaks im Zusammenhang mit hybridisierten Orbitalzuständen zeigten eine polare, von Ga dominierte Oberfläche. Außerdem zeigt sich eine Verringerung der Bandbiegung (BB) an der mit N_2^+ gesputterten n-GaN(0001) Oberfläche. Die Funktionalisierung der gesputterten n-GaN(0001) Oberfläche mit H2O Molekülen führte zu einer merklichen Veränderung ihrer Chemie und Bandbiegung (BB), wie aus den gemessenen NAP-XPS O 1s und Ga 3d Linien und den NAP-XPS VB Spektren der anschließend gesputterten n-GaN(0001)/H2O Oberfläche hervorgeht. Die Bildung von Ga-OH und N-H Bindungen sowie von Ga-O Bindungen, die bei hohen Temperaturen hervortreten, sind Merkmale, die aus den NAP-XPS Peaks vorhandener Oberflächenbestandteile abgeleitet werden. Eine solche Kompensation von freien Bindungen und Oberflächenzuständen bei zunehmender H2O Exposition äußerte sich als kontinuierliche Abnahme der BB Spannung der Oberfläche (V_bb) bis zu einem Druck p=0.1 mbar, oberhalb derer sie sich, abhängig von der Probentemperatur (T), auf einem konstanten Wert einpendelte. Die BB Spannung der Oberfläche V_bb wurde mit zwei Methoden abgeschätzt. Ein Ansatz nutzte die Energiedifferenz (E_FS-E_VS) zwischen der Oberflächenvalenzbandkante (E_VS) und dem Fermi-Niveau (E_FS), die VBM Energie, durch die lineare Extrapolationsmethode. Der andere alternative Ansatz beruht auf der gemessenen Übergangsenergie der Ga 3dS Linie der gesputterten n-GaN(0001) Oberfläche und der mit N_2^+ gesputterten n-GaN(0001)/H2O Grenzfläche, nämlich (E_FS-E_Ga3dS), kombiniert mit 17.5 eV, der Energiedifferenz zwischen VB Kante und Ga 3dB CL der GaN Masse. Die beiden Ansätze berücksichtigen die Energieunterschiede zwischen der Kante des Leitungsbandes (CB) (E_CB) und der Fermi-Energie E_FB des GaN Festkörpers (E_CB-E_FB) sowie zwischen E_CB und E_VB, der Bandlückenenergie E_g, die beide temperaturabhängig sind. Die beobachtete Abnahme von V_bb mit zunehmender Temperatur wird teilweise dadurch erklärt. Aufgrund der Schwierigkeit, eine exakte lineare Extrapolation für die VB Spektren der gesputterten n-GaN(0001)/H2O Grenzfläche, hauptsächlich bei hohem H2O Druck, zu finden, konnte V_bb sinnvoll nur aus den Verschiebungen von (E_FS-E_Ga3dS ), in Kombination mit (E_VB-E_Ga3dB )=17.5 eV, und (E_FS-E_VS ), der VBM Energie, geschätzt werden. Bei konstanter Temperatur wird die Abnahme von V_bb an der gesputterten n-GaN(0001)/H2O Grenzfläche bei einem Druck p unter 0,1 mbar auf die Kompensation von freien Bindungen an der Oberfläche durch dissoziative H2O Moleküle zurückgeführt. Die Konstanz von V_bb bei p>0.1 mbar wird als Sättigungsbedeckung der n-GaN(0001) Oberfläche mit H2O Molekülen verstanden. Der Rückgang von niedrig-Druck V_bb mit steigender Temperatur lässt sich zum Teil durch die Abnahme von E_g (T)=(E_CB-E_VB ) und (E_CB-E_FB (T)) mit steigender Temperatur erklären, obwohl ein solcher Rückgang nicht allein den gesamten Trend von V_bb bei hoher Temperatur erklären kann. Die gesputterte n-GaN(0001) Oberfläche ermöglicht eine bemerkenswerte Adhäsion von Molekülen der schwefelhaltigen Aminosäure L-Cystein (L-CySH), wie aus den jeweiligen UHV-XPS Spektren hervorgeht. Die Ablagerung von L-CySH auf der n-GaN(0001) Oberfläche erfolgte durch Sublimation von L-CySH Pulver bei 105 °C für ≤ 40 Minuten. Die UHV-XPS Spektren wurden an einer gesputterten GaN(0001)/L-Cystein Grenzfläche bei Raumtemperatur und nach Tempern bei verschiedenen Temperaturen (T_A=75-150℃) für Temperzeiten (t_A) von 15, 30 und 60 Minuten aufgenommen. Die UHV-XPS Spektren der gesputterten n-GaN(0001)/L-Cystein Grenzfläche zeigten, dass die Thiol Kopfgruppe (SH Gruppe) des L-Cystein Moleküls eine starke chemische Bindung mit den nicht gesättigten Bindungen des metallischen Ga an der Oberfläche einging. Ein solches Verhalten wurde anhand der überlappenden Ga 3s und S 2p XPS Photoemissionslinien beobachtet, deren Intensität durch mehr abgelagerte L-Cystein Schichten verstärkt wurde, ein Merkmal, das charakteristisch für Thiolatspezies ist. Der Bindung von Carboxyl- (COO-) und Aminogruppen (NH2) an der Oberfläche kann nicht vollständig ausgeschlossen werden, wie aus den jeweiligen C 1s, O 1s und N 1s XPS Spektren hervorgeht. Die Stärke und das Erscheinungsbild der Dublett S 2p Photoemissionslinien nahmen mit zunehmender T_A ab, was darauf hindeutet, dass die meisten, aber nicht alle L-Cystein Schichten, wieder von der funktionalisierten Oberfläche verdampften, wobei die SH Gruppe der untersten L-Cystein Schicht immer noch chemisch an die Ga Atome der Oberfläche gebunden war."]},{"key":"dc:title","label":"Title","values":["Characterization of bio-hybrid interfaces under ambient conditions"]}]}],"canonical_facts":{"dc:contributor.advisor":["Esser, Norbert"],"dc:creator":["Ahmad, Mais"],"dc:date.accessioned":["2024-04-10T10:44:52Z"],"dc:date.available":["2024-04-10T10:44:52Z"],"dc:date.issued":["2024"],"dc:description.abstract":["The topography, morphology, and chemistry of the surface of HVPE-grown n-GaN(0001) layers on c-sapphire/n-GaN(0001) samples were explored under various conditions before and after sputtering with 1 keV nitrogen N2 ions, followed by the functionalization with deionized water (H2O) and neutral L-cysteine (C3H7NO2S) molecules. Atomic force microscopy (AFM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) in ultra-high vacuum (UHV) and near-ambient-pressure (NAP) conditions were used for this investigation. Monochromatic X-ray 𝐴𝐴𝐴𝐴 𝐾𝐾𝐾𝐾 radi ation (ℎ𝜋𝜋 = 1486.7𝑒𝑒𝑉𝑉) was the exc itat ion source for the XPS experiments. The results obtained yielded information on the modifications occurring at the surface of the n-GaN(0001) layers upon N2+-sputtering and on the contact formation and interfacial chemical reactivity after exposure to H2O and L-cysteine, i.e., on the ensuing interactions and bonding mechanisms between H2O and L-cysteine molecules and the N2+-sputtered n-GaN(0001) surface. This provided a clue on the suitability, stability, and functionality of GaN-OH and GaN-SH groups, the primary links between the surface and inorganic, organic and biomolecules, and hence, on the potentiality of using GaN-based surfaces for electronic devices and biosensors. Experimental UHV-XPS spectra of C1s and O1s core-level (CL) transitions of the as-grown n-GaN(0001) surface revealed adsorbed adventitious carbon (C) and oxygen (O2) species, besides traces of Ga2O3 and Ga2Ox oxides. The surface AFM images and LEED patterns revealed that N2+-ion sputtering resulted in minimal surface damage and roughness, in addition to largely reducing the amount of adsorbed carbon species. The NAP-XPS spectra of the Ga 2p3/2 and Ga 3d5/2 photoemission lines of the N2+-sputtered n-GaN(0001)/H2O interface showed that the H2O molecules adsorbed on the n-GaN(0001) surface likely dissociated into a hydroxyl ion (OH−) and a proton (H+), which tend to combine with surface gallium (Ga+) dangling bonds and unbonded nitrogen (N−) ions, respectively. The N2+-ion sputtering and H2O exposure modified the valence band (VB) structure of the as-grown n-GaN(0001) surface. This is shown by the respective UHV and NAP-XPS VB photoemission spectra, which displayed VB peaks related to the hybridized orbital states of apolar surface dominated by Ga, besides revealing a reduction in the band bending at the N_2^+-sputtered n-GaN(0001) surface. Functionalization of the N_2^+-sputtered n-GaN(0001) surface with H2O molecules induced a noticeable modification to its chemistry and band bending (BB), disclosed from the measured NAP-XPS O 1s and Ga 3d lines, and from the NAP-XPS VB spectra of the ensuing N_2^+-sputtered n-GaN(0001)/H2O interface. The formation of Ga-OH and N-H bonds, and also Ga-O bonds that become prominent at high temperatures, are inferred from the NAP-XPS peaks of the existing surface constituents. Such a compensation of dangling bonds and surface states upon increasing the amount of H2O exposure typified itself as a continuous decrease in the surface BB voltage V_bb up to a pressure p=0.1 mbar, above which V_bb leveled off at a constant value, depending on the sample temperature T. The surface BB voltage V_bb was estimated by two methodologies. One approach used the energy difference (E_FS-E_VS) between the surface valence-band edge (E_VS) and the Fermi level (E_FS), the VBM energy, by the linear extrapolation method. The other approach monitored the measured peak energy of the Ga 3dS line of the N_2^+-sputtered n-GaN(0001) surface and the N_2^+-sputtered n-GaN(0001)/H2O interface viz., (E_FS-E_Ga3dS), combined with 17.5 eV, the energy difference between VB edge and Ga 3dB CL of bulk GaN. The two approaches incorporated the energy differences between the conduction band (CB) edge (E_CB) and the Fermi energy E_FB of bulk GaN (E_CB-E_FB) and between E_CB and E_VB, the band gap energy E_g, both of which are temperature dependent. This partially explains the observed decrease in V_bb with increasing T. Due to the difficulty of finding a proper linear extrapolation for the VB spectra of the N_2^+-sputtered n-GaN(0001)/H2O interface, mainly at high H2O pressure, V_bb was estimated from the binding energy shifts of (E_FS-E_Ga3dS ), combined with (E_VB-E_Ga3dB )=17.5eV, and (E_FS-E_VS ), the VBM energy. At a constant T, the decrease in V_bb at the N_2^+-sputtered n-GaN(0001)/H2O interface with pressure p below 0.1 mbar is ascribed to the compensation of surface dangling bonds by dissociative H2O molecules. The constancy of V_bb for p>0.1 mbar is understood in terms of a saturation coverage of the n-GaN(0001) surface with H2O molecules. The reduction of p>0.1 V_bb with increasing T can be partly accounted for by the decrease in E_g (T)=(E_CB-E_VB ) and (E_CB-E_FB (T)) with T, though such a decrease cannot solely clarify the entire trend of V_bb at high T. The N_2^+-sputtered n-GaN(0001) surface permits notable bonding of the sulfur-containing amino-acid L-cysteine (L-CySH) molecules, as concluded from the respective UHV-XPS spectra. Deposition of L-CySH on the n-GaN(0001) surface was made via sublimation of L-CySH powder at 105 oC for ≤ 40 min. The UHV-XPS spectra of the N_2^+-sputtered GaN(0001)/L-cysteine interface were taken at room temperature and after annealing at various temperatures (T_A=75-150℃) for annealing times (t_A) of 15, 30, and 60 minutes. The UHV-XPS spectra of the N_2^+-sputtered n-GaN(0001)/L-cysteine interface showed that the thiol head group (SH group) of the L-cysteine molecule formed strong chemical bonding with the surface metallic Ga. Such behavior was noted from the overlapped Ga 3s and S 2p XPS photoemission lines, the intensity of which were enhanced with more deposited L-cysteine layers, a feature that is characteristic for thiolate species. The bonding of carboxyl (COOH) and amino (NH2) groups to the surface cannot be entirely excluded, as was noted from the respective C 1s, O 1s, and N 1s XPS spectra. The strength and appearance of the doublet S 2p photoemission lines diminished as T_A increased, suggesting that most but not all L-cysteine layers re-evaporated from the functionalized surface, with the SH-group of the lowermost L-cysteine layer being still chemically bonded to the Ga atoms at the surface.","Die Topographie, Morphologie und Chemie der Oberfläche von HVPE-gewachsenen n-GaN(0001) Schichten auf c-Saphir/n-GaN(0001) Proben wurden vor und nach dem Sputtern mit 1 keV Stickstoffionen (N_2^+) und der Funktionalisierung mit entionisiertem Wasser (H2O) und neutralen L-Cystein Molekülen (C3H7NO2S) unter verschiedenen Bedingungen untersucht. Für diese Untersuchung wurden die Rasterkraftmikroskopie (AFM), die Niederenergieelektronenbeugung (LEED) und die Röntgenphotoelektronenspektroskopie (XPS) im Ultrahochvakuum (UHV) und in einer niedrig-Druck Atmosphäre (Near-Ambient-Pressure, NAP) angewendet. Die monochromatische Al-Kα Röntgenstrahlung (hν=1486.7eV) wurde bei den XPS Experimenten als Anregungsquelle eingesetzt. Die Ergebnisse lieferten Information über die durch das Sputtern verursachten Modifikationen an der Oberfläche der n-GaN(0001) Schichten, über die Kontaktbildung und die chemische Reaktivität an der Grenzfläche nach Einwirkung von H2O und L-Cystein, sowie zu die daraus resultierenden Wechselwirkungen und Bindungsmechanismen zwischen H2O und L-Cystein Molekülen und der gesputterten n-GaN(0001) Oberfläche. Dies lieferte einen Hinweis für die Eignung, Stabilität und Funktionalität von GaN-OH und GaN-SH Gruppen, den primären Verbindungen zwischen der Oberfläche und anorganischen organischen und Biomolekülen, und daher auch die Möglichkeit, GaN-basierte Oberflächen für elektronische Geräte und Biosensoren zu verwenden. Experimentelle UHV-XPS Spektren der Kernschalenübergänge C 1s und O 1s der unbehandelten n-GaN(0001) Oberfläche zeigten adsorbierte unspezifische Kohlenstoff- (C) und Sauerstoffspezies (O2) sowie Spuren von Ga2O3 und Ga2Ox Oxiden. Die AFM Bilder und LEED Muster der Oberfläche zeigten, dass das Sputtern mit N_2^+ Ionen nur minimale Oberflächenschäden und Rauheiten erzeugte, und zusätzlich die Anzahl der adsorbierten Kohlenstoffspezies deutlich verringerte. Die NAP-XPS Spektren der Ga 2p3/2 und Ga 3d5/2 Photoemissionslinien der gesputterten n-GaN(0001)/H2O Grenzfläche zeigten, dass auf der n-GaN(0001) Oberfläche adsorbierte H2O Moleküle wahrscheinlich in ein Hydroxyl Ion (OH-) und Proton (H+) dissoziierten, Ionen, die dazu neigen, sich mit ungesättigten Gallium (Ga^+) Bindungen bzw. ungebundenen Stickstoff (N^-) Ionen an der Oberfläche zu verbinden. Das Sputtern mit N_2^+ Ionen und die Abdeckung der Oberfläche durch H2O veränderten die Valenzbandstruktur (VB) der n-GaN(0001) Oberfläche, wie sie aus den jeweiligen UHV- und NAP-XPS VB Photoemissionsspektren hervorgeht. Die VB Peaks im Zusammenhang mit hybridisierten Orbitalzuständen zeigten eine polare, von Ga dominierte Oberfläche. Außerdem zeigt sich eine Verringerung der Bandbiegung (BB) an der mit N_2^+ gesputterten n-GaN(0001) Oberfläche. Die Funktionalisierung der gesputterten n-GaN(0001) Oberfläche mit H2O Molekülen führte zu einer merklichen Veränderung ihrer Chemie und Bandbiegung (BB), wie aus den gemessenen NAP-XPS O 1s und Ga 3d Linien und den NAP-XPS VB Spektren der anschließend gesputterten n-GaN(0001)/H2O Oberfläche hervorgeht. Die Bildung von Ga-OH und N-H Bindungen sowie von Ga-O Bindungen, die bei hohen Temperaturen hervortreten, sind Merkmale, die aus den NAP-XPS Peaks vorhandener Oberflächenbestandteile abgeleitet werden. Eine solche Kompensation von freien Bindungen und Oberflächenzuständen bei zunehmender H2O Exposition äußerte sich als kontinuierliche Abnahme der BB Spannung der Oberfläche (V_bb) bis zu einem Druck p=0.1 mbar, oberhalb derer sie sich, abhängig von der Probentemperatur (T), auf einem konstanten Wert einpendelte. Die BB Spannung der Oberfläche V_bb wurde mit zwei Methoden abgeschätzt. Ein Ansatz nutzte die Energiedifferenz (E_FS-E_VS) zwischen der Oberflächenvalenzbandkante (E_VS) und dem Fermi-Niveau (E_FS), die VBM Energie, durch die lineare Extrapolationsmethode. Der andere alternative Ansatz beruht auf der gemessenen Übergangsenergie der Ga 3dS Linie der gesputterten n-GaN(0001) Oberfläche und der mit N_2^+ gesputterten n-GaN(0001)/H2O Grenzfläche, nämlich (E_FS-E_Ga3dS), kombiniert mit 17.5 eV, der Energiedifferenz zwischen VB Kante und Ga 3dB CL der GaN Masse. Die beiden Ansätze berücksichtigen die Energieunterschiede zwischen der Kante des Leitungsbandes (CB) (E_CB) und der Fermi-Energie E_FB des GaN Festkörpers (E_CB-E_FB) sowie zwischen E_CB und E_VB, der Bandlückenenergie E_g, die beide temperaturabhängig sind. Die beobachtete Abnahme von V_bb mit zunehmender Temperatur wird teilweise dadurch erklärt. Aufgrund der Schwierigkeit, eine exakte lineare Extrapolation für die VB Spektren der gesputterten n-GaN(0001)/H2O Grenzfläche, hauptsächlich bei hohem H2O Druck, zu finden, konnte V_bb sinnvoll nur aus den Verschiebungen von (E_FS-E_Ga3dS ), in Kombination mit (E_VB-E_Ga3dB )=17.5 eV, und (E_FS-E_VS ), der VBM Energie, geschätzt werden. Bei konstanter Temperatur wird die Abnahme von V_bb an der gesputterten n-GaN(0001)/H2O Grenzfläche bei einem Druck p unter 0,1 mbar auf die Kompensation von freien Bindungen an der Oberfläche durch dissoziative H2O Moleküle zurückgeführt. Die Konstanz von V_bb bei p>0.1 mbar wird als Sättigungsbedeckung der n-GaN(0001) Oberfläche mit H2O Molekülen verstanden. Der Rückgang von niedrig-Druck V_bb mit steigender Temperatur lässt sich zum Teil durch die Abnahme von E_g (T)=(E_CB-E_VB ) und (E_CB-E_FB (T)) mit steigender Temperatur erklären, obwohl ein solcher Rückgang nicht allein den gesamten Trend von V_bb bei hoher Temperatur erklären kann. Die gesputterte n-GaN(0001) Oberfläche ermöglicht eine bemerkenswerte Adhäsion von Molekülen der schwefelhaltigen Aminosäure L-Cystein (L-CySH), wie aus den jeweiligen UHV-XPS Spektren hervorgeht. Die Ablagerung von L-CySH auf der n-GaN(0001) Oberfläche erfolgte durch Sublimation von L-CySH Pulver bei 105 °C für ≤ 40 Minuten. Die UHV-XPS Spektren wurden an einer gesputterten GaN(0001)/L-Cystein Grenzfläche bei Raumtemperatur und nach Tempern bei verschiedenen Temperaturen (T_A=75-150℃) für Temperzeiten (t_A) von 15, 30 und 60 Minuten aufgenommen. Die UHV-XPS Spektren der gesputterten n-GaN(0001)/L-Cystein Grenzfläche zeigten, dass die Thiol Kopfgruppe (SH Gruppe) des L-Cystein Moleküls eine starke chemische Bindung mit den nicht gesättigten Bindungen des metallischen Ga an der Oberfläche einging. Ein solches Verhalten wurde anhand der überlappenden Ga 3s und S 2p XPS Photoemissionslinien beobachtet, deren Intensität durch mehr abgelagerte L-Cystein Schichten verstärkt wurde, ein Merkmal, das charakteristisch für Thiolatspezies ist. Der Bindung von Carboxyl- (COO-) und Aminogruppen (NH2) an der Oberfläche kann nicht vollständig ausgeschlossen werden, wie aus den jeweiligen C 1s, O 1s und N 1s XPS Spektren hervorgeht. Die Stärke und das Erscheinungsbild der Dublett S 2p Photoemissionslinien nahmen mit zunehmender T_A ab, was darauf hindeutet, dass die meisten, aber nicht alle L-Cystein Schichten, wieder von der funktionalisierten Oberfläche verdampften, wobei die SH Gruppe der untersten L-Cystein Schicht immer noch chemisch an die Ga Atome der Oberfläche gebunden war."],"dc:identifier.uri":["https://depositonce.tu-berlin.de/handle/11303/21299","https://doi.org/10.14279/depositonce-20099"],"dc:language.iso":["en"],"dc:rights.uri":["https://creativecommons.org/licenses/by/4.0/"],"dc:title":["Characterization of bio-hybrid interfaces under ambient conditions"],"dc:type":["Doctoral Thesis"]},"updated_at":"2026-07-27T21:28:52Z"}