Two-dimensional (2D) materials can have multiple phases close in energy but with distinct propert... more Two-dimensional (2D) materials can have multiple phases close in energy but with distinct properties, with the phases that form during growth dependent on experimental conditions and the growth substrate. Here,...
Physical chemistry chemical physics : PCCP, Jan 13, 2017
The ability to affect the surface properties of non-polar Cr2O3 films through polar ZnO(0001) and... more The ability to affect the surface properties of non-polar Cr2O3 films through polar ZnO(0001) and (0001[combining macron]) supports was investigated by characterizing the polarity of ZnO films grown on top of the Cr2O3 surfaces. The growth and geometric and electronic structures of the ZnO films were characterized with X-ray photoelectron spectroscopy, ultra-violet photoelectron spectroscopy, reflection high-energy electron diffraction, low-energy electron diffraction, and X-ray diffraction. The ZnO growth mode was Stranski-Krastanov, which can be attributed to the ZnO layers initially adopting a non-polar structure with a lower surface tension before transitioning to the polar bulk structure with a higher surface energy. A similar result has been reported for ZnO growth on α-Al2O3(0001), which is isostructural with Cr2O3. The polarity of the added ZnO layer was determined by examining the surface morphology following wet chemical etching with atomic force microscopy and by characte...
Physical chemistry chemical physics : PCCP, Jan 31, 2017
Two-dimensional (2D) silica (SiO2) and aluminosilicate (AlSi3O8) bilayers grown on Pd(111) were f... more Two-dimensional (2D) silica (SiO2) and aluminosilicate (AlSi3O8) bilayers grown on Pd(111) were fabricated and systematically studied using ultrahigh vacuum surface analysis in combination with theoretical methods, including Auger electron spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory. Based on LEED results, both SiO2 and AlSi3O8 bilayers start ordering above 850 K in 2 × 10(-6) Torr oxygen. Both bilayers show hexagonal LEED patterns with a periodicity approximately twice that of the Pd(111) surface. Importantly, the SiO2 bilayer forms an incommensurate crystalline structure whereas the AlSi3O8 bilayer crystallizes in a commensurate structure. The incommensurate crystalline SiO2 structure on Pd(111) resulted in a moiré pattern observed with LEED and STM. Theoretical results show that straining the pure SiO2 bilayer to match Pd(111) would cost 0.492 eV per unit cell; this stra...
Epitaxial strain can be a powerful parameter for directing the growth of thin films. Unfortunatel... more Epitaxial strain can be a powerful parameter for directing the growth of thin films. Unfortunately, conventional materials only offer discrete choices for setting the lattice strain. In this work, it is demonstrated that epitaxial growth of transition metal alloy solid solutions can provide thermally stable, high-quality growth substrates with continuously tunable lattice constants. Molecular beam epitaxy was used to grow NixPd1-x(111) alloy films with lattice constants between 3.61 and 3.89 Å on the hexagonal (0001) basal planes of α-Al2O3 and Cr2O3 (grown as epitaxial films on α-Al2O3 (0001)). The Cr2O3 acted as an adhesion layer, which not only improved the high-temperature stability of the films but also produced single-domain films with NixPd1-x [112̅] parallel to Cr2O3 [112̅0], in contrast to growth on α-Al2O3 that yielded twinned films. Surface characterization by electron diffraction and scanning tunneling microscopy (STM) as well as bulk X-ray diffraction analysis indicated...
a b s t r a c t Cd(OH) 2 nanoparticles (NPs) were synthesized on graphite treated with plasma thr... more a b s t r a c t Cd(OH) 2 nanoparticles (NPs) were synthesized on graphite treated with plasma through electrochemical deposition in a solution of CdF 2 saturated with O 2 . Cd(OH) 2 NPs were formed from reaction of Cd 2+ in solution with OH − generated on the graphite after reduction of O 2 . The Cd(OH) 2 NPs were densely distributed on the surface and converted to form hollow CdS NPs via a thermal reaction with H 2 S at 300 • C. The resulting Cd(OH) 2 and CdS NPs were characterized with measurements of X-ray photoelectron spectroscopy, atomic force microscopy, and high-resolution transmission electron microscopy. The latter results showed that the Cd(OH) 2 and CdS NPs exhibited hexagonal and cubic crystalline structures, respectively. The photoluminescence of CdS NP showed a direct band at 2.47 eV and an indirect band at ∼1.67 eV.
We investigated the adsorption and reaction of methanol on continuous and discontinuous films of ... more We investigated the adsorption and reaction of methanol on continuous and discontinuous films of samarium oxide (SmO x ) grown on Pt(111) in ultrahigh vacuum. The methanol decomposition was studied by temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), while structural changes of the oxide surface were monitored by low-energy electron diffraction (LEED). Methanol dehydrogenates to adsorbed methoxy species on both the continuous and discontinuous SmO x films, eventually leading to the desorption of CO and H 2 which desorbs at temperatures in the range 400-600 K. Small quantities of CO 2 are also detected mainly on as-prepared Sm 2 O 3 thin films, but the production of CO 2 is limited during repeated TPD runs. The discontinuous film exhibits the highest reactivity compared to the continuous film and the Pt(111) substrate. The reactivity of methanol on reduced and reoxidized films was also investigated, revealing how SmO x structures influence the chemical behavior. Over repeated TPD experiments, a SmO x structural/chemical equilibrium condition is found which can be approached either from oxidized or reduced films. We also observed hydrogen absence in TPD which indicates that hydrogen is stored either in SmO x films or as OH groups on the SmO x surfaces. Materials 2015, 8 6229
Physical chemistry chemical physics : PCCP, Jan 23, 2015
Rare earth praseodymium and cerium oxides have attracted intense research interest in the last fe... more Rare earth praseodymium and cerium oxides have attracted intense research interest in the last few decades, due to their intriguing chemical and physical characteristics. An understanding of the correlation between structure and properties, in particular the surface chemistry, is urgently required for their application in microelectronics, catalysis, optics and other fields. Such an understanding is, however, hampered by the complexity of rare earth oxide materials and experimental methods for their characterisation. Here, we report recent progress in studying high-quality, single crystalline, praseodymium and cerium oxide films as well as ternary alloys grown on Si(111) substrates. Using these well-defined systems and based on a systematic multi-technique surface science approach, the corresponding physical and chemical properties, such as the surface structure, the surface morphology, the bulk-surface interaction and the oxygen storage/release capability, are explored in detail. W...
ABSTRACT We investigated the growth and partial reduction of Sm2O3(111) thin films on Pt(111) usi... more ABSTRACT We investigated the growth and partial reduction of Sm2O3(111) thin films on Pt(111) using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find that the Sm2O3(111) films are high quality and grow in a defective fluorite structure wherein the Sm cations adopt a hexagonal (1.37 × 1.37) lattice in registry with the Pt(111) surface, while oxygen vacancies are randomly distributed within the film. STM measurements show that Sm2O3(111) film growth on Pt(111) occurs by the Stranski-Krastanov mechanism, in which a single O–Sm–O trilayer initially forms, followed by the growth of well-defined, multilayer islands. The Sm2O3(111) films undergo partial reduction during annealing at 1000 K in ultrahigh vacuum. LEED and STM provide evidence that a fraction of the Sm2O3 in the first layer, closest to the Pt(111) substrate, decomposes to produce well-ordered domains of rocksalt SmO(100) during reduction, and that Sm2O3 from the third and higher layers concurrently spreads onto the first layer to form a more contiguous second layer of Sm2O3(111). We show that the SmO(100) and Sm2O3(111) lattices can form a coincidence structure with minimal strain to the Sm-atom sublattices, and that satellite features observed in the LEED patterns are consistent with the coexistence of SmO(100) and Sm2O3(111) domains as well as the proposed Sm2O3(111)/SmO(100) coincidence structure. Lastly, we find that reoxidation of the partially reduced films restores the original Sm2O3(111) crystal structure, and significantly improves the film quality, as reflected by a flatter film morphology and better connectivity among oxide domains. An implication from this study is that the formation of (100)-oriented monoxide structures is a general characteristic of the reduction of rare-earth oxide thin films on hexagonally close-packed metal surfaces.
ABSTRACT The inherent properties of epitaxial oxide thin-film layers have attracted the intense i... more ABSTRACT The inherent properties of epitaxial oxide thin-film layers have attracted the intense interest of different research fields, such as catalysis and microelectronics. The focus of this work is the temperature-dependent oxygen release, oxygen vacancy formation, and lattice rearrangement of Ce1–xPrxO2−δ thin films with systematic stoichiometry variation (x = 0–1) and oxygen deficiency (δ > 0) on Si(111). The mixed oxide layers were heteroepitaxially grown by coevaporating molecular beam epitaxy. To observe the oxygen release, temperature-programmed desorption was performed. Furthermore, laboratory-based X-ray diffraction measurements were carried out after several annealing steps to investigate the crystal structure rearrangement. The contribution of Ce4+/Ce3+ and Pr4+/Pr3+ redox systems to the oxygen release and lattice rearrangement was clarified by X-ray photoelectron spectroscopy. Finally, Raman spectroscopy was performed to detect structural defects in the oxide lattice (i.e., oxygen vacancies and MO8-complexes) and their temperature dependence, which thus provides microscopic insights into the atomic oxygen release mechanism. The oxygen-releasing temperature and the oxygen storage capacity in such Ce1–xPrxO2−δ model thin films can be engineered by the Pr concentration.
ABSTRACT 4-Alkylaryl layers were electrodeposited on Au surface via formation in situ of diazoniu... more ABSTRACT 4-Alkylaryl layers were electrodeposited on Au surface via formation in situ of diazonium cations and the subsequent electroreduction in solution of 4-alkylanilines (4-CH3(CH2)C6H4NH2, n = 5-13). The electrodeposited layers were characterized with X-ray photoelectron spectra (XPS), an atomic force microscope (AFM), attenuated-total-reflection infrared spectra (ATR-IR), spectroscopic ellipsometry, and water contact angle. The thickness of the deposited alkylaryl layer increased with the length of the alkyl chain, which was about 1.6 times the molecular length of the alkylaniline precursor. The surfaces of deposited adlayers were uniform and smooth without aggregation and exhibited hydrophobic character with a water contact angle similar to 95 degrees. Charge transport across the alkylaryl layer was characterized with a measurement of a conductive AFM. The current-voltage characteristics at low voltages are describable with a nonresonant tunneling mechanism, according to which the resistance increased exponentially with the thickness of the adlayer. As a measure of the efficiency of the charge transport, the average attenuation factor (beta) was 0.63 angstrom(-1), which was less than that observed for the alkyl SAM because of the presence of the aryl ring. The breakdown voltage of the adlayer also increased with the length of the alkyl chain. The 4-tetradocylaryl layer exhibited a resistance and breakdown voltage comparable to that observed for a 1-octadecanethiolate SAM.
a b s t r a c t Cd(OH) 2 nanoparticles (NPs) were synthesized on graphite treated with plasma thr... more a b s t r a c t Cd(OH) 2 nanoparticles (NPs) were synthesized on graphite treated with plasma through electrochemical deposition in a solution of CdF 2 saturated with O 2 . Cd(OH) 2 NPs were formed from reaction of Cd 2+ in solution with OH − generated on the graphite after reduction of O 2 . The Cd(OH) 2 NPs were densely distributed on the surface and converted to form hollow CdS NPs via a thermal reaction with H 2 S at 300 • C. The resulting Cd(OH) 2 and CdS NPs were characterized with measurements of X-ray photoelectron spectroscopy, atomic force microscopy, and high-resolution transmission electron microscopy. The latter results showed that the Cd(OH) 2 and CdS NPs exhibited hexagonal and cubic crystalline structures, respectively. The photoluminescence of CdS NP showed a direct band at 2.47 eV and an indirect band at ∼1.67 eV.
ABSTRACT 4-Alkylaryl layers were electrodeposited on Au surface via formation in situ of diazoniu... more ABSTRACT 4-Alkylaryl layers were electrodeposited on Au surface via formation in situ of diazonium cations and the subsequent electroreduction in solution of 4-alkylanilines (4-CH3(CH2)C6H4NH2, n = 5-13). The electrodeposited layers were characterized with X-ray photoelectron spectra (XPS), an atomic force microscope (AFM), attenuated-total-reflection infrared spectra (ATR-IR), spectroscopic ellipsometry, and water contact angle. The thickness of the deposited alkylaryl layer increased with the length of the alkyl chain, which was about 1.6 times the molecular length of the alkylaniline precursor. The surfaces of deposited adlayers were uniform and smooth without aggregation and exhibited hydrophobic character with a water contact angle similar to 95 degrees. Charge transport across the alkylaryl layer was characterized with a measurement of a conductive AFM. The current-voltage characteristics at low voltages are describable with a nonresonant tunneling mechanism, according to which the resistance increased exponentially with the thickness of the adlayer. As a measure of the efficiency of the charge transport, the average attenuation factor (beta) was 0.63 angstrom(-1), which was less than that observed for the alkyl SAM because of the presence of the aryl ring. The breakdown voltage of the adlayer also increased with the length of the alkyl chain. The 4-tetradocylaryl layer exhibited a resistance and breakdown voltage comparable to that observed for a 1-octadecanethiolate SAM.
Two-dimensional (2D) materials can have multiple phases close in energy but with distinct propert... more Two-dimensional (2D) materials can have multiple phases close in energy but with distinct properties, with the phases that form during growth dependent on experimental conditions and the growth substrate. Here,...
Physical chemistry chemical physics : PCCP, Jan 13, 2017
The ability to affect the surface properties of non-polar Cr2O3 films through polar ZnO(0001) and... more The ability to affect the surface properties of non-polar Cr2O3 films through polar ZnO(0001) and (0001[combining macron]) supports was investigated by characterizing the polarity of ZnO films grown on top of the Cr2O3 surfaces. The growth and geometric and electronic structures of the ZnO films were characterized with X-ray photoelectron spectroscopy, ultra-violet photoelectron spectroscopy, reflection high-energy electron diffraction, low-energy electron diffraction, and X-ray diffraction. The ZnO growth mode was Stranski-Krastanov, which can be attributed to the ZnO layers initially adopting a non-polar structure with a lower surface tension before transitioning to the polar bulk structure with a higher surface energy. A similar result has been reported for ZnO growth on α-Al2O3(0001), which is isostructural with Cr2O3. The polarity of the added ZnO layer was determined by examining the surface morphology following wet chemical etching with atomic force microscopy and by characte...
Physical chemistry chemical physics : PCCP, Jan 31, 2017
Two-dimensional (2D) silica (SiO2) and aluminosilicate (AlSi3O8) bilayers grown on Pd(111) were f... more Two-dimensional (2D) silica (SiO2) and aluminosilicate (AlSi3O8) bilayers grown on Pd(111) were fabricated and systematically studied using ultrahigh vacuum surface analysis in combination with theoretical methods, including Auger electron spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory. Based on LEED results, both SiO2 and AlSi3O8 bilayers start ordering above 850 K in 2 × 10(-6) Torr oxygen. Both bilayers show hexagonal LEED patterns with a periodicity approximately twice that of the Pd(111) surface. Importantly, the SiO2 bilayer forms an incommensurate crystalline structure whereas the AlSi3O8 bilayer crystallizes in a commensurate structure. The incommensurate crystalline SiO2 structure on Pd(111) resulted in a moiré pattern observed with LEED and STM. Theoretical results show that straining the pure SiO2 bilayer to match Pd(111) would cost 0.492 eV per unit cell; this stra...
Epitaxial strain can be a powerful parameter for directing the growth of thin films. Unfortunatel... more Epitaxial strain can be a powerful parameter for directing the growth of thin films. Unfortunately, conventional materials only offer discrete choices for setting the lattice strain. In this work, it is demonstrated that epitaxial growth of transition metal alloy solid solutions can provide thermally stable, high-quality growth substrates with continuously tunable lattice constants. Molecular beam epitaxy was used to grow NixPd1-x(111) alloy films with lattice constants between 3.61 and 3.89 Å on the hexagonal (0001) basal planes of α-Al2O3 and Cr2O3 (grown as epitaxial films on α-Al2O3 (0001)). The Cr2O3 acted as an adhesion layer, which not only improved the high-temperature stability of the films but also produced single-domain films with NixPd1-x [112̅] parallel to Cr2O3 [112̅0], in contrast to growth on α-Al2O3 that yielded twinned films. Surface characterization by electron diffraction and scanning tunneling microscopy (STM) as well as bulk X-ray diffraction analysis indicated...
a b s t r a c t Cd(OH) 2 nanoparticles (NPs) were synthesized on graphite treated with plasma thr... more a b s t r a c t Cd(OH) 2 nanoparticles (NPs) were synthesized on graphite treated with plasma through electrochemical deposition in a solution of CdF 2 saturated with O 2 . Cd(OH) 2 NPs were formed from reaction of Cd 2+ in solution with OH − generated on the graphite after reduction of O 2 . The Cd(OH) 2 NPs were densely distributed on the surface and converted to form hollow CdS NPs via a thermal reaction with H 2 S at 300 • C. The resulting Cd(OH) 2 and CdS NPs were characterized with measurements of X-ray photoelectron spectroscopy, atomic force microscopy, and high-resolution transmission electron microscopy. The latter results showed that the Cd(OH) 2 and CdS NPs exhibited hexagonal and cubic crystalline structures, respectively. The photoluminescence of CdS NP showed a direct band at 2.47 eV and an indirect band at ∼1.67 eV.
We investigated the adsorption and reaction of methanol on continuous and discontinuous films of ... more We investigated the adsorption and reaction of methanol on continuous and discontinuous films of samarium oxide (SmO x ) grown on Pt(111) in ultrahigh vacuum. The methanol decomposition was studied by temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), while structural changes of the oxide surface were monitored by low-energy electron diffraction (LEED). Methanol dehydrogenates to adsorbed methoxy species on both the continuous and discontinuous SmO x films, eventually leading to the desorption of CO and H 2 which desorbs at temperatures in the range 400-600 K. Small quantities of CO 2 are also detected mainly on as-prepared Sm 2 O 3 thin films, but the production of CO 2 is limited during repeated TPD runs. The discontinuous film exhibits the highest reactivity compared to the continuous film and the Pt(111) substrate. The reactivity of methanol on reduced and reoxidized films was also investigated, revealing how SmO x structures influence the chemical behavior. Over repeated TPD experiments, a SmO x structural/chemical equilibrium condition is found which can be approached either from oxidized or reduced films. We also observed hydrogen absence in TPD which indicates that hydrogen is stored either in SmO x films or as OH groups on the SmO x surfaces. Materials 2015, 8 6229
Physical chemistry chemical physics : PCCP, Jan 23, 2015
Rare earth praseodymium and cerium oxides have attracted intense research interest in the last fe... more Rare earth praseodymium and cerium oxides have attracted intense research interest in the last few decades, due to their intriguing chemical and physical characteristics. An understanding of the correlation between structure and properties, in particular the surface chemistry, is urgently required for their application in microelectronics, catalysis, optics and other fields. Such an understanding is, however, hampered by the complexity of rare earth oxide materials and experimental methods for their characterisation. Here, we report recent progress in studying high-quality, single crystalline, praseodymium and cerium oxide films as well as ternary alloys grown on Si(111) substrates. Using these well-defined systems and based on a systematic multi-technique surface science approach, the corresponding physical and chemical properties, such as the surface structure, the surface morphology, the bulk-surface interaction and the oxygen storage/release capability, are explored in detail. W...
ABSTRACT We investigated the growth and partial reduction of Sm2O3(111) thin films on Pt(111) usi... more ABSTRACT We investigated the growth and partial reduction of Sm2O3(111) thin films on Pt(111) using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find that the Sm2O3(111) films are high quality and grow in a defective fluorite structure wherein the Sm cations adopt a hexagonal (1.37 × 1.37) lattice in registry with the Pt(111) surface, while oxygen vacancies are randomly distributed within the film. STM measurements show that Sm2O3(111) film growth on Pt(111) occurs by the Stranski-Krastanov mechanism, in which a single O–Sm–O trilayer initially forms, followed by the growth of well-defined, multilayer islands. The Sm2O3(111) films undergo partial reduction during annealing at 1000 K in ultrahigh vacuum. LEED and STM provide evidence that a fraction of the Sm2O3 in the first layer, closest to the Pt(111) substrate, decomposes to produce well-ordered domains of rocksalt SmO(100) during reduction, and that Sm2O3 from the third and higher layers concurrently spreads onto the first layer to form a more contiguous second layer of Sm2O3(111). We show that the SmO(100) and Sm2O3(111) lattices can form a coincidence structure with minimal strain to the Sm-atom sublattices, and that satellite features observed in the LEED patterns are consistent with the coexistence of SmO(100) and Sm2O3(111) domains as well as the proposed Sm2O3(111)/SmO(100) coincidence structure. Lastly, we find that reoxidation of the partially reduced films restores the original Sm2O3(111) crystal structure, and significantly improves the film quality, as reflected by a flatter film morphology and better connectivity among oxide domains. An implication from this study is that the formation of (100)-oriented monoxide structures is a general characteristic of the reduction of rare-earth oxide thin films on hexagonally close-packed metal surfaces.
ABSTRACT The inherent properties of epitaxial oxide thin-film layers have attracted the intense i... more ABSTRACT The inherent properties of epitaxial oxide thin-film layers have attracted the intense interest of different research fields, such as catalysis and microelectronics. The focus of this work is the temperature-dependent oxygen release, oxygen vacancy formation, and lattice rearrangement of Ce1–xPrxO2−δ thin films with systematic stoichiometry variation (x = 0–1) and oxygen deficiency (δ > 0) on Si(111). The mixed oxide layers were heteroepitaxially grown by coevaporating molecular beam epitaxy. To observe the oxygen release, temperature-programmed desorption was performed. Furthermore, laboratory-based X-ray diffraction measurements were carried out after several annealing steps to investigate the crystal structure rearrangement. The contribution of Ce4+/Ce3+ and Pr4+/Pr3+ redox systems to the oxygen release and lattice rearrangement was clarified by X-ray photoelectron spectroscopy. Finally, Raman spectroscopy was performed to detect structural defects in the oxide lattice (i.e., oxygen vacancies and MO8-complexes) and their temperature dependence, which thus provides microscopic insights into the atomic oxygen release mechanism. The oxygen-releasing temperature and the oxygen storage capacity in such Ce1–xPrxO2−δ model thin films can be engineered by the Pr concentration.
ABSTRACT 4-Alkylaryl layers were electrodeposited on Au surface via formation in situ of diazoniu... more ABSTRACT 4-Alkylaryl layers were electrodeposited on Au surface via formation in situ of diazonium cations and the subsequent electroreduction in solution of 4-alkylanilines (4-CH3(CH2)C6H4NH2, n = 5-13). The electrodeposited layers were characterized with X-ray photoelectron spectra (XPS), an atomic force microscope (AFM), attenuated-total-reflection infrared spectra (ATR-IR), spectroscopic ellipsometry, and water contact angle. The thickness of the deposited alkylaryl layer increased with the length of the alkyl chain, which was about 1.6 times the molecular length of the alkylaniline precursor. The surfaces of deposited adlayers were uniform and smooth without aggregation and exhibited hydrophobic character with a water contact angle similar to 95 degrees. Charge transport across the alkylaryl layer was characterized with a measurement of a conductive AFM. The current-voltage characteristics at low voltages are describable with a nonresonant tunneling mechanism, according to which the resistance increased exponentially with the thickness of the adlayer. As a measure of the efficiency of the charge transport, the average attenuation factor (beta) was 0.63 angstrom(-1), which was less than that observed for the alkyl SAM because of the presence of the aryl ring. The breakdown voltage of the adlayer also increased with the length of the alkyl chain. The 4-tetradocylaryl layer exhibited a resistance and breakdown voltage comparable to that observed for a 1-octadecanethiolate SAM.
a b s t r a c t Cd(OH) 2 nanoparticles (NPs) were synthesized on graphite treated with plasma thr... more a b s t r a c t Cd(OH) 2 nanoparticles (NPs) were synthesized on graphite treated with plasma through electrochemical deposition in a solution of CdF 2 saturated with O 2 . Cd(OH) 2 NPs were formed from reaction of Cd 2+ in solution with OH − generated on the graphite after reduction of O 2 . The Cd(OH) 2 NPs were densely distributed on the surface and converted to form hollow CdS NPs via a thermal reaction with H 2 S at 300 • C. The resulting Cd(OH) 2 and CdS NPs were characterized with measurements of X-ray photoelectron spectroscopy, atomic force microscopy, and high-resolution transmission electron microscopy. The latter results showed that the Cd(OH) 2 and CdS NPs exhibited hexagonal and cubic crystalline structures, respectively. The photoluminescence of CdS NP showed a direct band at 2.47 eV and an indirect band at ∼1.67 eV.
ABSTRACT 4-Alkylaryl layers were electrodeposited on Au surface via formation in situ of diazoniu... more ABSTRACT 4-Alkylaryl layers were electrodeposited on Au surface via formation in situ of diazonium cations and the subsequent electroreduction in solution of 4-alkylanilines (4-CH3(CH2)C6H4NH2, n = 5-13). The electrodeposited layers were characterized with X-ray photoelectron spectra (XPS), an atomic force microscope (AFM), attenuated-total-reflection infrared spectra (ATR-IR), spectroscopic ellipsometry, and water contact angle. The thickness of the deposited alkylaryl layer increased with the length of the alkyl chain, which was about 1.6 times the molecular length of the alkylaniline precursor. The surfaces of deposited adlayers were uniform and smooth without aggregation and exhibited hydrophobic character with a water contact angle similar to 95 degrees. Charge transport across the alkylaryl layer was characterized with a measurement of a conductive AFM. The current-voltage characteristics at low voltages are describable with a nonresonant tunneling mechanism, according to which the resistance increased exponentially with the thickness of the adlayer. As a measure of the efficiency of the charge transport, the average attenuation factor (beta) was 0.63 angstrom(-1), which was less than that observed for the alkyl SAM because of the presence of the aryl ring. The breakdown voltage of the adlayer also increased with the length of the alkyl chain. The 4-tetradocylaryl layer exhibited a resistance and breakdown voltage comparable to that observed for a 1-octadecanethiolate SAM.
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Papers by JinHao Jhang