To understand the atomistic phenomenon behind initial oxidation processes, we have studied the na... more To understand the atomistic phenomenon behind initial oxidation processes, we have studied the nanoscale evolution of oxide growth prior to the formation of a complete layer on a Ni-15 wt%Cr(100) alloy surface using scanning tunneling microscopy/ spectroscopy (STM/STS). At the onset of oxidation, a NiO superlattice forms oxide wedges across the step edges, eventually growing across the terraces. The completion of the NiO layer is followed by nucleation of the next layer, which always commences at the groove site of the superlattice. The Cr-oxide formation initiates as disk-shaped oxide particles early in the oxidation process, which Monte Carlo simulations reveal are likely caused by Cr clustering across the alloy surface. Upon further oxidation, a Cr(100)-p(2 × 2)O reconstructed surface is observed, indicating phase separation of Cr predicates the formation of the passive Cr-oxide film. The STS results vary across the oxide-alloy interface and between each oxide, providing greater insight into the origins of electronic heterogeneity and their effect on oxide growth. Using these data, we propose an oxidation model that highlights the growth of partial oxide layers on Ni-Cr(100) alloys within the pre-Cabrera-Mott regime.
The investigation of C 60-Au interaction is central to the advancement of solar cell and nanotech... more The investigation of C 60-Au interaction is central to the advancement of solar cell and nanotechnology applications of C 60. C 60 grows in a quasi-layer-by-layer mode on a pristine graphite surface and form a special surface structure (coexistence of round and fractal islands). The deposition of Au leads to the formation of a complex array of different surface structures, while the basic island structure of the C 60 is preserved. The Au-clusters nucleate preferentially at the graphite-first fullerene layer islands edge forming beadlike structures. A roughness analysis of the fullerene surface indicates the presence of Au atoms embedded in the fullerene surface, situated in the troughs in between the large molecules. The analysis of the spatial and size distributions of Au clusters provides the basis for the development of a qualitative model which describes the relevant surface processes in the Aufullerene system. The simultaneous deposition of Au and C 60 leads to the formation of organized structures, in which Au clusters are embedded in a ring of fullerene molecules with a constant width.
The study of thin film magnetic materials and the doping of semiconductors with magnetically acti... more The study of thin film magnetic materials and the doping of semiconductors with magnetically active dopant atoms has received increased attention due their potential applications in magnetic memory devices and spintronics. We observe the deposition of Mn on the Si(100) 2x1 reconstructed surface in the sub-monolayer regime with STM. Short Mn wires with a length of 5 to about 20 atoms are formed an oriented perpendicular to the Si-dimer rows. At higher coverage some Mn wires are anchored with one end of the wire at the edge and extend onto the lower lying surface. The region in between the wires is particularly interesting: if the Mn wire distance is reduced the dimers change their orientation and are tilted, or begin to form zig-zag lines. The wire length and dimer deformation is likely governed by local strain. We will discuss the wire statistics (lengths, orientation, and position), control of their growth and present first data on the electronic structure of the wires. The growth ...
Ni-Cr and Ni-Cr-Mo alloys owe their outstanding corrosion resistance to the surface enrichment of... more Ni-Cr and Ni-Cr-Mo alloys owe their outstanding corrosion resistance to the surface enrichment of passivating Cr(III) oxides and synergistic effect of Cr and Mo in case of Mo-containing alloys [1-3]. However, the specific roles of minor elements are not well understood especially with respect to precise location of Mo relative to the oxide/metal interface, nor the atomistic processes responsible for protective-oxide layer growth and breakdown. The composition, structure and thickness of the passivating oxide films are challenging to characterize considering their nanoscale dimensions and the high electric field imposed during growth in solution. Key processes that take place within the oxide and regulate passivation are controlled by defect interactions that are atomic, ionic, and electronic in nature and currently poorly understood, often needed to be studied at the resolution and detection limits of experimental methods [4]. Common electrochemical methods such as AC and DC electro...
Intercalation of C 60 molecules at the graphenesubstrate interface by annealing leads to amorphou... more Intercalation of C 60 molecules at the graphenesubstrate interface by annealing leads to amorphous and crystalline structures. A comparison of topography and electronic structure with wrinkles and moirépatterns confirms intercalation. The intercalated molecules imprint a local strain/deformation on the graphene layer whose magnitude is controlled by the intermolecular distance. The crystalline intercalated structure exhibits a superlattice peak in the local density of states. This work provides control of local strain in graphene.
The effects of crystal orientation and prior etching on the polarization and repassivation behavi... more The effects of crystal orientation and prior etching on the polarization and repassivation behavior of Ni− Cr and Ni−Cr−Mo alloys have been investigated in acidic chloride environment using dc potentiostatic and ac singlefrequency electrochemical impedance spectroscopy. Tests were conducted within the passive region at potentials where local oxide breakdown was possible. Surface morphologies of grains across a wide range of orientations were measured before and after passivation using atomic force microscopy. Oxide growth was monitored on isolated lowand high-index crystallographic planes as a function of repassivation time, enabling the independent measurement of the oxidation and total anodic current densities. Grains exhibited the tendency to either passivate with significant or minimal oxidation or instead resist active passivation and repassivation. The oxidation performance was a function of the crystallographic orientation of the exposed grain surface. The oxides grown on various-orientated surfaces differed in their film growth kinetics, morphology, and steady state thicknesses, even given similar total anodic current densities due to dissimilar oxidation efficiencies. For Ni-11 wt % Cr, only orientations close to (1 0 1) were able to form stable passive films, aided by nanofaceted surfaces with a matchstick-type morphology. On the other hand, all orientations of the Ni-11 wt % Cr-6 wt % Mo alloys formed electrochemically stable oxides films owing to the beneficial influence of Mo on repassivation. Passivation, breakdown behavior, and oxide properties vary with crystal orientation, and this work provides insight into the effects of crystallographic orientation and oxide film morphology on the passivation mechanism of Ni−Cr and Ni−Cr−Mo alloys.
The annealing of sub-critical Ge wetting layers (WL < 3.5 ML) initiates the formation of 3D na... more The annealing of sub-critical Ge wetting layers (WL < 3.5 ML) initiates the formation of 3D nanostructures, whose shape and orientation is determined by the WL thickness and thus directly related to the strain energy. The emergence of these nanostructures, hillocks and pre-quantum dots, is studied by scanning tunneling microscopy. A wetting layer deposited at 350 °C is initially rough on the nanometer length-scale and undergoes a progressive transformation and smoothening during annealing at T < 460 °C when vacancy lines and the 2xn reconstruction are observed. The metastable Ge WL then collapses to form 3D nanostructures whose morphology is controlled by the WL thickness: first, the hillocks, with a wedding cake-type structure where the step edges run parallel to the ⟨110⟩ direction, are formed from thin wetting layers, while {105}-faceted structures, called pre-quantum dots (p-QDs), are formed from thicker layers. The wetting layer thickness and thus the misfit strain energy...
Dilute magnetic semiconductors are important building blocks towards the realization of low power... more Dilute magnetic semiconductors are important building blocks towards the realization of low power, low thermal loss spintronics based devices. In this work, group IV based dilute magnetic semiconductors are investigated for their potential to function at temperature above room temperature and compatibility with the current microelectronics industry. The goal of this work
The effects of substrate treatment, growth temperature, and composition on the surface morphology... more The effects of substrate treatment, growth temperature, and composition on the surface morphology of Ni-Cr thin films grown on MgO(001) are studied by scanning tunneling microscopy and atomic force microscopy. We demonstrate that a combination of acid-etched substrates and high temperature deposition (400 °C) will result in smooth films with well-defined terraces (up to 30 nm wide) that are suitable for the study of progression of chemical reactions on the surface. Two different treatments are used to prepare the MgO substrates for deposition and they introduce characteristic differences in film surface morphology. Thin films that are grown on the phosphoric acid-treated substrates present reduced nucleation density during the initial stages of film growth which results in long and wide terraces. Due to the ≈16% lattice mismatch in the Ni(001)/MgO(001) system, film growth at 400 °C yields discontinuous films and a two-step growth process is necessary to obtain a continuous layer. Ni...
Bulletin of the American Physical Society, Mar 4, 2015
Metallic nanoparticles are highly coveted but are subject to rapid Ostwald ripening even at moder... more Metallic nanoparticles are highly coveted but are subject to rapid Ostwald ripening even at moderate temperatures limiting study of their properties. Ultrasmall transition metal carbide "nanospheres" are synthesized by a solid-state reaction between fullerene as carbon scaffold, and a W surface. This produces nanospheres with a narrow size distribution below 2.5 nm diameter. The nanosphere shape is defined by the scaffold and densely packed arrays can be achieved. The metal-fullerene reaction is temperature driven and progresses through an intermediate semiconducting phase until the fully metallic nanospheres are created at about 350 C. The reaction sequence is observed with STM, and STS maps yield the local density of states. The reaction presumably progresses by stepwise introduction of W-atoms in the carbon scaffold. The results of high resolution STM/STS in combination with DFT calculations are used to unravel the reaction mechanism. We will discuss the transfer of this specific reaction mechanism to other transition metal carbides. The nanospheres are an excellent testbed for the physics and chemistry of highly curved surfaces.
In the present work we describe an investigation of the nucleation mechanism of polycrystalline d... more In the present work we describe an investigation of the nucleation mechanism of polycrystalline diamond films if the bias-enhanced-nucleation ͑BEN͒ method is used. Photoelectron spectroscopy with excitation energies in the ultraviolet ͓ultraviolet photoelectron spectroscopy ͑UPS͔͒ and x-ray regime ͑x-ray photoelectron spectroscopy͒ as well as electron energy loss spectroscopy are employed to monitor the nucleation process and the subsequent diamond film growth. The deposition is performed in situ, thus avoiding surface contamination with oxygen or hydrocarbons. The observation of the temporal evolution of composition and structure of the deposited film and its interface with the underlying silicon substrate allow us to develop a qualitative model, which describes the nucleation process. The BEN pretreatment leads, through the irradiation with low-energy ions, to the codeposition of an amorphous carbon phase and the crystalline diamond phase. The presence of both phases is readily apparent in the UPS analysis, which will prove to be an indispensible tool in the structural characterization of the carbon phase present at the surface. There is no indication for the presence of graphite or large graphitic clusters. A deconvolution of the C 1s and Si 2p core-level peaks does confirm the presence of two carbon phases and the formation of a silicon carbide interface. With increasing deposition time the contribution of diamond to the carbon film increases and upon switching to diamond growth conditions the amorphous carbon phase is rapidly etched and only the diamond crystals remain and continue to grow. This removal of the amorphous phase leads to a decrease in the overall carbon concentration at the surface by 18-30 % during the first 30 sec of the diamond growth period and was observed for a variety of pretreatment conditions. A silicon carbide interfacial layer is formed early on during the BEN pretreatment and its thickness is reduced considerably by etching during the diamond growth period. These results are summarized and discussed in the framework of a qualitative model for the nucleation process. ͓S0163-1829͑97͒03628-X͔
In this paper the properties of diamond/amorphous carbon heterostructures are studied using photo... more In this paper the properties of diamond/amorphous carbon heterostructures are studied using photoelectron spectroscopy in the ultraviolet ͑UPS͒ and x-ray ͑XPS͒ regime. The nondiamond carbon films are deposited on a p-doped polycrystalline diamond substrate, and produced by first, electron beam evaporation of graphite forming hydrogen-free amorphous carbon ͑a-C͒ and, second, ion beam deposition, producing amorphous hydrogenated carbon (a-C:H) films. The overlayer formation is monitored step-by-step, and the changes in band bending in the diamond substrate and the valence band discontinuities are deduced from the UPS and XPS spectra. In the diamond/a-C structure a downward band bending in diamond evolves continuously with overlayer thickness and a final value of about 1.1 eV is obtained, resulting in a band offset of 1.5Ϯ0.1 eV. In the diamond/a-C:H structure a downward band bending of 1.4 eV is observed after only a brief deposition time, when the estimated overlayer thickness is still less than a monolayer. The ion energies employed for the film deposition were 200 and 600 eV and although the resulting overlayers exhibit characteristic structural differences, the band bending and band offset (1.4Ϯ0.15 eV) are not noticeably influenced. It appears likely that in this case the defects formed in the diamond lattice through the energetic ions used for the overlayer deposition, are responsible for a pinning of the Fermi level at the surface. In an additional experiment the diamond substrate was irradiated with Ar ϩ ions ͑3 keV͒ and the resulting band bending, induced through the creation of a defect-rich surface layer, amounts to about 1.3 eV. The introduction of dangling bonds and/or-bonded regions, which are energetically located in the gap of diamond, leads to the observed increase in downward band bending in the p-doped diamond. ͓S0163-1829͑99͒13847-5͔
The growth of Si clusters on the C 60 surface was observed in situ using photoelectron spectrosco... more The growth of Si clusters on the C 60 surface was observed in situ using photoelectron spectroscopy in the ultraviolet ͑UPS͒ and x-ray regimes. The highly corrugated C 60 surface provides a wide range of bonding sites and might function as a template for the formation of cluster superstructures. The Si accumulates first in the interstitial bonding sites, which present a bonding environment defined by the fullerene molecules and dominated by carbon atoms. This arrangement is comparable to the formation of a surface fulleride and the interstitial sites apparently act as primary nucleation sites. The clusters then continue to grow and the Siu Si bonding environment gains in importance. A numerical fit of the core level spectra ͑C1s and Si2p͒ reveals the relative contributions of the different bonding environments as the cluster growth proceeds. The absence of a SiC contribution excludes a substitutional doping of the fullerene cage with Si. The impact of initial and final state effects due to changes in the photoemission process as the cluster size is varied, can be identified and separated from the influence of the chemical shift ͑variation in bonding environment͒ on the core level position and shape. The UPS valence band spectra confirm the absence of a charge transfer or directed covalent bonding between the Si adsorbate and the fullerene surface. The valence band offset between the amorphous Si and the C 60 amounts to 1.2 eV, and the position of the C1s core level is likely pinned by the a-Si dangling bonds. The C 60 can function as a template for the formation of Si cluster arrays ͑superstructures͒ with the interstitial sites as primary nucleation sites and an expansion to three-dimensional structures is possible.
In the present work we describe an investigation of the influence of low-energy ion irradiation ͑... more In the present work we describe an investigation of the influence of low-energy ion irradiation ͑1-keV Ar ϩ ͒ on the surface structure of polycrystalline diamond and amorphous carbon films with various degrees of graphitization. Photoelectron spectroscopy ͑PES͒ with excitation energies in the ultraviolet and x-ray regime is employed to monitor the radiation-induced modification of the electronic structure of the surface which is closely linked to the local bonding environment of the carbon atoms. A comparison of the mean photoelectron escape depth and the thickness of the irradiation affected layer also illustrates the suitability of PES for this investigation. For the chemical vapor deposition ͑CVD͒-diamond film a gradual change from typical diamond features to amorphous carbon is observed for ion doses surpassing 6ϫ10 14 cm Ϫ2. The structural changes in the diamond lattice are expressed in a broadening of the C 1s core-level peak, and increasing contributions from p-states around 3-4 eV in the valence-band spectra. Likewise the peak located at 13 eV ͑He II, hϭ40.82 eV͒, characteristic of diamond, is no longer apparent for ion doses exceeding 3ϫ10 15 cm Ϫ2. The diamond surface clearly shows a tendency to amorphize rather than graphitize under ion irradiation. To complement the results for the diamond film, we irradiated two amorphous carbon films with different microstructures: ͑A͒ a predominantly amorphous film, and ͑B͒ a film with graphitic inclusions. The destruction of graphitic structures in film ͑B͒ is apparent even for the lowest ion dose ͑1.5ϫ10 14 cm Ϫ2 ͒, and expressed in an increase in the width of the C 1s core-level peak and a smearing out of the valence-band spectral features. The resultant valenceband and core-level spectra ͑total ion dose: 3ϫ10 15 cm Ϫ2 ͒ are very similar to the one obtained for the amorphous film ͑A͒, which, on the other hand, does not show any significant changes in its structure upon irradiation. The irradiation of both diamond and graphitic structures with low-energy Ar ϩ ions leads to the formation of a predominantly amorphous surface layer. ͓S0163-1829͑96͒07934-9͔
Metal clusters with less than 50 atoms, are of great interest in the development of true nanoscal... more Metal clusters with less than 50 atoms, are of great interest in the development of true nanoscale electronics. The electronic structure is strongly size dependent and future applications rely on a narrow size distribution. Our goal was to investigate the possibility to use fullerene surfaces as templates in the formation of cluster arrays, to analyze the cluster-fullerene interface and the electronic structure of the components. This study was performed by using photoelectron spectroscopy, and scanning tunneling microscopy and spectroscopy (UHV-conditions). The Au-cluster size is controlled by the coverage and the fullerene lattice limits the cluster surface mobility, making them accessible to STM analysis. The fullerene matrix enhances the cluster stability and ripening occurs above 500 K. A comprehensive description of the Au-cluster-fullerene system has been achieved and includes the interface characteristics, local and global electronic structure, and the spatial distribution o...
ginia-The electronic properties of graphene can be modified through the formation of a charge or ... more ginia-The electronic properties of graphene can be modified through the formation of a charge or topographic superlattice, in our study this is achieved by intercalation of fullerene molecules at the interface between copper and graphene. Amorphous and crystalline superlattices can be synthesized and are controlled by annealing T (650 K to 850 K) and time. The crystalline superlattices present a square geometry defined by the Cu(001) facet and the period can be controlled by deposition conditions. The geometric and electronic structure of the superlattice is measured with STM (scanning tunneling microscopy), ST spectroscopy and differential conductivity maps. The intercalation of C60 is confirmed by (i) atomic resolution of graphene on top of molecule, (ii) spectral signature of graphene is modulated with shoulder at 250 meV, (iii) bias voltage dependence of apparent height, and (iv) depth between molecules correlates with intermolecule distance due to mechanical deformation of graphene. The crystalline layer imprints a charge superlattice with 1.5 holes/molecule donated to graphene-while the graphene is nearly neutral in between. The intercalation is a versatile method to control superlattice formation with potential for tuning charge carrier transport. 1 We acknowledge the support by NSF DMR-100580.
The combination of Silicon with an element with a large magnetic moment such as Manganese is high... more The combination of Silicon with an element with a large magnetic moment such as Manganese is highly desirable for the development of novel spintronics devices. We present a study on the surface-driven synthesis of Mn-nanostructures on the Si(100) (2x1) surface using STM and photoelectron spectroscopy. The Si-surface functions as a template and monoatomic Mn-nanowires are formed, which always run perpendicular
The combination of Si and Ge with Mn is a critical step in the development of novel spintronics d... more The combination of Si and Ge with Mn is a critical step in the development of novel spintronics devices. We investigate the magnetic doping of Si, Si-surfaces and Ge- quantum dots with Mn. A surface-driven route is used for the addition of Mn and allows a stringent control of the Mn-Si and Mn-Ge interaction. The evolution of nanostructures is observed
Fullerene thin films are deposited on defect-rich Highly Oriented Pyrolitic Graphite (HOPG) and i... more Fullerene thin films are deposited on defect-rich Highly Oriented Pyrolitic Graphite (HOPG) and investigated by scanning tunneling microscopy (STM). These films exhibit a wealth of novel structural and electronic features which are not present in fullerene layers deposited on pristine graphite surfaces. Molecule nanostructures comprised of single molecules with a greatly enhanced apparent size, trimer structures, and hexamer structures are observed. The geometry of these molecular structures is linked to the stacking of the C60 layer, and ...
To understand the atomistic phenomenon behind initial oxidation processes, we have studied the na... more To understand the atomistic phenomenon behind initial oxidation processes, we have studied the nanoscale evolution of oxide growth prior to the formation of a complete layer on a Ni-15 wt%Cr(100) alloy surface using scanning tunneling microscopy/ spectroscopy (STM/STS). At the onset of oxidation, a NiO superlattice forms oxide wedges across the step edges, eventually growing across the terraces. The completion of the NiO layer is followed by nucleation of the next layer, which always commences at the groove site of the superlattice. The Cr-oxide formation initiates as disk-shaped oxide particles early in the oxidation process, which Monte Carlo simulations reveal are likely caused by Cr clustering across the alloy surface. Upon further oxidation, a Cr(100)-p(2 × 2)O reconstructed surface is observed, indicating phase separation of Cr predicates the formation of the passive Cr-oxide film. The STS results vary across the oxide-alloy interface and between each oxide, providing greater insight into the origins of electronic heterogeneity and their effect on oxide growth. Using these data, we propose an oxidation model that highlights the growth of partial oxide layers on Ni-Cr(100) alloys within the pre-Cabrera-Mott regime.
The investigation of C 60-Au interaction is central to the advancement of solar cell and nanotech... more The investigation of C 60-Au interaction is central to the advancement of solar cell and nanotechnology applications of C 60. C 60 grows in a quasi-layer-by-layer mode on a pristine graphite surface and form a special surface structure (coexistence of round and fractal islands). The deposition of Au leads to the formation of a complex array of different surface structures, while the basic island structure of the C 60 is preserved. The Au-clusters nucleate preferentially at the graphite-first fullerene layer islands edge forming beadlike structures. A roughness analysis of the fullerene surface indicates the presence of Au atoms embedded in the fullerene surface, situated in the troughs in between the large molecules. The analysis of the spatial and size distributions of Au clusters provides the basis for the development of a qualitative model which describes the relevant surface processes in the Aufullerene system. The simultaneous deposition of Au and C 60 leads to the formation of organized structures, in which Au clusters are embedded in a ring of fullerene molecules with a constant width.
The study of thin film magnetic materials and the doping of semiconductors with magnetically acti... more The study of thin film magnetic materials and the doping of semiconductors with magnetically active dopant atoms has received increased attention due their potential applications in magnetic memory devices and spintronics. We observe the deposition of Mn on the Si(100) 2x1 reconstructed surface in the sub-monolayer regime with STM. Short Mn wires with a length of 5 to about 20 atoms are formed an oriented perpendicular to the Si-dimer rows. At higher coverage some Mn wires are anchored with one end of the wire at the edge and extend onto the lower lying surface. The region in between the wires is particularly interesting: if the Mn wire distance is reduced the dimers change their orientation and are tilted, or begin to form zig-zag lines. The wire length and dimer deformation is likely governed by local strain. We will discuss the wire statistics (lengths, orientation, and position), control of their growth and present first data on the electronic structure of the wires. The growth ...
Ni-Cr and Ni-Cr-Mo alloys owe their outstanding corrosion resistance to the surface enrichment of... more Ni-Cr and Ni-Cr-Mo alloys owe their outstanding corrosion resistance to the surface enrichment of passivating Cr(III) oxides and synergistic effect of Cr and Mo in case of Mo-containing alloys [1-3]. However, the specific roles of minor elements are not well understood especially with respect to precise location of Mo relative to the oxide/metal interface, nor the atomistic processes responsible for protective-oxide layer growth and breakdown. The composition, structure and thickness of the passivating oxide films are challenging to characterize considering their nanoscale dimensions and the high electric field imposed during growth in solution. Key processes that take place within the oxide and regulate passivation are controlled by defect interactions that are atomic, ionic, and electronic in nature and currently poorly understood, often needed to be studied at the resolution and detection limits of experimental methods [4]. Common electrochemical methods such as AC and DC electro...
Intercalation of C 60 molecules at the graphenesubstrate interface by annealing leads to amorphou... more Intercalation of C 60 molecules at the graphenesubstrate interface by annealing leads to amorphous and crystalline structures. A comparison of topography and electronic structure with wrinkles and moirépatterns confirms intercalation. The intercalated molecules imprint a local strain/deformation on the graphene layer whose magnitude is controlled by the intermolecular distance. The crystalline intercalated structure exhibits a superlattice peak in the local density of states. This work provides control of local strain in graphene.
The effects of crystal orientation and prior etching on the polarization and repassivation behavi... more The effects of crystal orientation and prior etching on the polarization and repassivation behavior of Ni− Cr and Ni−Cr−Mo alloys have been investigated in acidic chloride environment using dc potentiostatic and ac singlefrequency electrochemical impedance spectroscopy. Tests were conducted within the passive region at potentials where local oxide breakdown was possible. Surface morphologies of grains across a wide range of orientations were measured before and after passivation using atomic force microscopy. Oxide growth was monitored on isolated lowand high-index crystallographic planes as a function of repassivation time, enabling the independent measurement of the oxidation and total anodic current densities. Grains exhibited the tendency to either passivate with significant or minimal oxidation or instead resist active passivation and repassivation. The oxidation performance was a function of the crystallographic orientation of the exposed grain surface. The oxides grown on various-orientated surfaces differed in their film growth kinetics, morphology, and steady state thicknesses, even given similar total anodic current densities due to dissimilar oxidation efficiencies. For Ni-11 wt % Cr, only orientations close to (1 0 1) were able to form stable passive films, aided by nanofaceted surfaces with a matchstick-type morphology. On the other hand, all orientations of the Ni-11 wt % Cr-6 wt % Mo alloys formed electrochemically stable oxides films owing to the beneficial influence of Mo on repassivation. Passivation, breakdown behavior, and oxide properties vary with crystal orientation, and this work provides insight into the effects of crystallographic orientation and oxide film morphology on the passivation mechanism of Ni−Cr and Ni−Cr−Mo alloys.
The annealing of sub-critical Ge wetting layers (WL < 3.5 ML) initiates the formation of 3D na... more The annealing of sub-critical Ge wetting layers (WL < 3.5 ML) initiates the formation of 3D nanostructures, whose shape and orientation is determined by the WL thickness and thus directly related to the strain energy. The emergence of these nanostructures, hillocks and pre-quantum dots, is studied by scanning tunneling microscopy. A wetting layer deposited at 350 °C is initially rough on the nanometer length-scale and undergoes a progressive transformation and smoothening during annealing at T < 460 °C when vacancy lines and the 2xn reconstruction are observed. The metastable Ge WL then collapses to form 3D nanostructures whose morphology is controlled by the WL thickness: first, the hillocks, with a wedding cake-type structure where the step edges run parallel to the ⟨110⟩ direction, are formed from thin wetting layers, while {105}-faceted structures, called pre-quantum dots (p-QDs), are formed from thicker layers. The wetting layer thickness and thus the misfit strain energy...
Dilute magnetic semiconductors are important building blocks towards the realization of low power... more Dilute magnetic semiconductors are important building blocks towards the realization of low power, low thermal loss spintronics based devices. In this work, group IV based dilute magnetic semiconductors are investigated for their potential to function at temperature above room temperature and compatibility with the current microelectronics industry. The goal of this work
The effects of substrate treatment, growth temperature, and composition on the surface morphology... more The effects of substrate treatment, growth temperature, and composition on the surface morphology of Ni-Cr thin films grown on MgO(001) are studied by scanning tunneling microscopy and atomic force microscopy. We demonstrate that a combination of acid-etched substrates and high temperature deposition (400 °C) will result in smooth films with well-defined terraces (up to 30 nm wide) that are suitable for the study of progression of chemical reactions on the surface. Two different treatments are used to prepare the MgO substrates for deposition and they introduce characteristic differences in film surface morphology. Thin films that are grown on the phosphoric acid-treated substrates present reduced nucleation density during the initial stages of film growth which results in long and wide terraces. Due to the ≈16% lattice mismatch in the Ni(001)/MgO(001) system, film growth at 400 °C yields discontinuous films and a two-step growth process is necessary to obtain a continuous layer. Ni...
Bulletin of the American Physical Society, Mar 4, 2015
Metallic nanoparticles are highly coveted but are subject to rapid Ostwald ripening even at moder... more Metallic nanoparticles are highly coveted but are subject to rapid Ostwald ripening even at moderate temperatures limiting study of their properties. Ultrasmall transition metal carbide "nanospheres" are synthesized by a solid-state reaction between fullerene as carbon scaffold, and a W surface. This produces nanospheres with a narrow size distribution below 2.5 nm diameter. The nanosphere shape is defined by the scaffold and densely packed arrays can be achieved. The metal-fullerene reaction is temperature driven and progresses through an intermediate semiconducting phase until the fully metallic nanospheres are created at about 350 C. The reaction sequence is observed with STM, and STS maps yield the local density of states. The reaction presumably progresses by stepwise introduction of W-atoms in the carbon scaffold. The results of high resolution STM/STS in combination with DFT calculations are used to unravel the reaction mechanism. We will discuss the transfer of this specific reaction mechanism to other transition metal carbides. The nanospheres are an excellent testbed for the physics and chemistry of highly curved surfaces.
In the present work we describe an investigation of the nucleation mechanism of polycrystalline d... more In the present work we describe an investigation of the nucleation mechanism of polycrystalline diamond films if the bias-enhanced-nucleation ͑BEN͒ method is used. Photoelectron spectroscopy with excitation energies in the ultraviolet ͓ultraviolet photoelectron spectroscopy ͑UPS͔͒ and x-ray regime ͑x-ray photoelectron spectroscopy͒ as well as electron energy loss spectroscopy are employed to monitor the nucleation process and the subsequent diamond film growth. The deposition is performed in situ, thus avoiding surface contamination with oxygen or hydrocarbons. The observation of the temporal evolution of composition and structure of the deposited film and its interface with the underlying silicon substrate allow us to develop a qualitative model, which describes the nucleation process. The BEN pretreatment leads, through the irradiation with low-energy ions, to the codeposition of an amorphous carbon phase and the crystalline diamond phase. The presence of both phases is readily apparent in the UPS analysis, which will prove to be an indispensible tool in the structural characterization of the carbon phase present at the surface. There is no indication for the presence of graphite or large graphitic clusters. A deconvolution of the C 1s and Si 2p core-level peaks does confirm the presence of two carbon phases and the formation of a silicon carbide interface. With increasing deposition time the contribution of diamond to the carbon film increases and upon switching to diamond growth conditions the amorphous carbon phase is rapidly etched and only the diamond crystals remain and continue to grow. This removal of the amorphous phase leads to a decrease in the overall carbon concentration at the surface by 18-30 % during the first 30 sec of the diamond growth period and was observed for a variety of pretreatment conditions. A silicon carbide interfacial layer is formed early on during the BEN pretreatment and its thickness is reduced considerably by etching during the diamond growth period. These results are summarized and discussed in the framework of a qualitative model for the nucleation process. ͓S0163-1829͑97͒03628-X͔
In this paper the properties of diamond/amorphous carbon heterostructures are studied using photo... more In this paper the properties of diamond/amorphous carbon heterostructures are studied using photoelectron spectroscopy in the ultraviolet ͑UPS͒ and x-ray ͑XPS͒ regime. The nondiamond carbon films are deposited on a p-doped polycrystalline diamond substrate, and produced by first, electron beam evaporation of graphite forming hydrogen-free amorphous carbon ͑a-C͒ and, second, ion beam deposition, producing amorphous hydrogenated carbon (a-C:H) films. The overlayer formation is monitored step-by-step, and the changes in band bending in the diamond substrate and the valence band discontinuities are deduced from the UPS and XPS spectra. In the diamond/a-C structure a downward band bending in diamond evolves continuously with overlayer thickness and a final value of about 1.1 eV is obtained, resulting in a band offset of 1.5Ϯ0.1 eV. In the diamond/a-C:H structure a downward band bending of 1.4 eV is observed after only a brief deposition time, when the estimated overlayer thickness is still less than a monolayer. The ion energies employed for the film deposition were 200 and 600 eV and although the resulting overlayers exhibit characteristic structural differences, the band bending and band offset (1.4Ϯ0.15 eV) are not noticeably influenced. It appears likely that in this case the defects formed in the diamond lattice through the energetic ions used for the overlayer deposition, are responsible for a pinning of the Fermi level at the surface. In an additional experiment the diamond substrate was irradiated with Ar ϩ ions ͑3 keV͒ and the resulting band bending, induced through the creation of a defect-rich surface layer, amounts to about 1.3 eV. The introduction of dangling bonds and/or-bonded regions, which are energetically located in the gap of diamond, leads to the observed increase in downward band bending in the p-doped diamond. ͓S0163-1829͑99͒13847-5͔
The growth of Si clusters on the C 60 surface was observed in situ using photoelectron spectrosco... more The growth of Si clusters on the C 60 surface was observed in situ using photoelectron spectroscopy in the ultraviolet ͑UPS͒ and x-ray regimes. The highly corrugated C 60 surface provides a wide range of bonding sites and might function as a template for the formation of cluster superstructures. The Si accumulates first in the interstitial bonding sites, which present a bonding environment defined by the fullerene molecules and dominated by carbon atoms. This arrangement is comparable to the formation of a surface fulleride and the interstitial sites apparently act as primary nucleation sites. The clusters then continue to grow and the Siu Si bonding environment gains in importance. A numerical fit of the core level spectra ͑C1s and Si2p͒ reveals the relative contributions of the different bonding environments as the cluster growth proceeds. The absence of a SiC contribution excludes a substitutional doping of the fullerene cage with Si. The impact of initial and final state effects due to changes in the photoemission process as the cluster size is varied, can be identified and separated from the influence of the chemical shift ͑variation in bonding environment͒ on the core level position and shape. The UPS valence band spectra confirm the absence of a charge transfer or directed covalent bonding between the Si adsorbate and the fullerene surface. The valence band offset between the amorphous Si and the C 60 amounts to 1.2 eV, and the position of the C1s core level is likely pinned by the a-Si dangling bonds. The C 60 can function as a template for the formation of Si cluster arrays ͑superstructures͒ with the interstitial sites as primary nucleation sites and an expansion to three-dimensional structures is possible.
In the present work we describe an investigation of the influence of low-energy ion irradiation ͑... more In the present work we describe an investigation of the influence of low-energy ion irradiation ͑1-keV Ar ϩ ͒ on the surface structure of polycrystalline diamond and amorphous carbon films with various degrees of graphitization. Photoelectron spectroscopy ͑PES͒ with excitation energies in the ultraviolet and x-ray regime is employed to monitor the radiation-induced modification of the electronic structure of the surface which is closely linked to the local bonding environment of the carbon atoms. A comparison of the mean photoelectron escape depth and the thickness of the irradiation affected layer also illustrates the suitability of PES for this investigation. For the chemical vapor deposition ͑CVD͒-diamond film a gradual change from typical diamond features to amorphous carbon is observed for ion doses surpassing 6ϫ10 14 cm Ϫ2. The structural changes in the diamond lattice are expressed in a broadening of the C 1s core-level peak, and increasing contributions from p-states around 3-4 eV in the valence-band spectra. Likewise the peak located at 13 eV ͑He II, hϭ40.82 eV͒, characteristic of diamond, is no longer apparent for ion doses exceeding 3ϫ10 15 cm Ϫ2. The diamond surface clearly shows a tendency to amorphize rather than graphitize under ion irradiation. To complement the results for the diamond film, we irradiated two amorphous carbon films with different microstructures: ͑A͒ a predominantly amorphous film, and ͑B͒ a film with graphitic inclusions. The destruction of graphitic structures in film ͑B͒ is apparent even for the lowest ion dose ͑1.5ϫ10 14 cm Ϫ2 ͒, and expressed in an increase in the width of the C 1s core-level peak and a smearing out of the valence-band spectral features. The resultant valenceband and core-level spectra ͑total ion dose: 3ϫ10 15 cm Ϫ2 ͒ are very similar to the one obtained for the amorphous film ͑A͒, which, on the other hand, does not show any significant changes in its structure upon irradiation. The irradiation of both diamond and graphitic structures with low-energy Ar ϩ ions leads to the formation of a predominantly amorphous surface layer. ͓S0163-1829͑96͒07934-9͔
Metal clusters with less than 50 atoms, are of great interest in the development of true nanoscal... more Metal clusters with less than 50 atoms, are of great interest in the development of true nanoscale electronics. The electronic structure is strongly size dependent and future applications rely on a narrow size distribution. Our goal was to investigate the possibility to use fullerene surfaces as templates in the formation of cluster arrays, to analyze the cluster-fullerene interface and the electronic structure of the components. This study was performed by using photoelectron spectroscopy, and scanning tunneling microscopy and spectroscopy (UHV-conditions). The Au-cluster size is controlled by the coverage and the fullerene lattice limits the cluster surface mobility, making them accessible to STM analysis. The fullerene matrix enhances the cluster stability and ripening occurs above 500 K. A comprehensive description of the Au-cluster-fullerene system has been achieved and includes the interface characteristics, local and global electronic structure, and the spatial distribution o...
ginia-The electronic properties of graphene can be modified through the formation of a charge or ... more ginia-The electronic properties of graphene can be modified through the formation of a charge or topographic superlattice, in our study this is achieved by intercalation of fullerene molecules at the interface between copper and graphene. Amorphous and crystalline superlattices can be synthesized and are controlled by annealing T (650 K to 850 K) and time. The crystalline superlattices present a square geometry defined by the Cu(001) facet and the period can be controlled by deposition conditions. The geometric and electronic structure of the superlattice is measured with STM (scanning tunneling microscopy), ST spectroscopy and differential conductivity maps. The intercalation of C60 is confirmed by (i) atomic resolution of graphene on top of molecule, (ii) spectral signature of graphene is modulated with shoulder at 250 meV, (iii) bias voltage dependence of apparent height, and (iv) depth between molecules correlates with intermolecule distance due to mechanical deformation of graphene. The crystalline layer imprints a charge superlattice with 1.5 holes/molecule donated to graphene-while the graphene is nearly neutral in between. The intercalation is a versatile method to control superlattice formation with potential for tuning charge carrier transport. 1 We acknowledge the support by NSF DMR-100580.
The combination of Silicon with an element with a large magnetic moment such as Manganese is high... more The combination of Silicon with an element with a large magnetic moment such as Manganese is highly desirable for the development of novel spintronics devices. We present a study on the surface-driven synthesis of Mn-nanostructures on the Si(100) (2x1) surface using STM and photoelectron spectroscopy. The Si-surface functions as a template and monoatomic Mn-nanowires are formed, which always run perpendicular
The combination of Si and Ge with Mn is a critical step in the development of novel spintronics d... more The combination of Si and Ge with Mn is a critical step in the development of novel spintronics devices. We investigate the magnetic doping of Si, Si-surfaces and Ge- quantum dots with Mn. A surface-driven route is used for the addition of Mn and allows a stringent control of the Mn-Si and Mn-Ge interaction. The evolution of nanostructures is observed
Fullerene thin films are deposited on defect-rich Highly Oriented Pyrolitic Graphite (HOPG) and i... more Fullerene thin films are deposited on defect-rich Highly Oriented Pyrolitic Graphite (HOPG) and investigated by scanning tunneling microscopy (STM). These films exhibit a wealth of novel structural and electronic features which are not present in fullerene layers deposited on pristine graphite surfaces. Molecule nanostructures comprised of single molecules with a greatly enhanced apparent size, trimer structures, and hexamer structures are observed. The geometry of these molecular structures is linked to the stacking of the C60 layer, and ...
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Papers by Petra Reinke