M. Enachescu - Academia.edu (original) (raw)

Papers by M. Enachescu

Research paper thumbnail of Observation of Coulomb staircase and negative differential resistance at room temperature by scanning tunneling microscopy

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1996

Nanometer-sized Au dots have been fabricated on p-type Si͑111͒ substrates by field-induced transf... more Nanometer-sized Au dots have been fabricated on p-type Si͑111͒ substrates by field-induced transfer of tip material in a scanning tunneling microscope, thus forming a nanometer Schottky contact. Positioning the scanning tunneling microscope tip on a very tiny dot ͑10-15 nm in diameter͒, equidistant steps with a spacing of several 100 mV and regions with negative differential resistance are observed in the current-voltage characteristic. Numerical simulations confirm that the determinant capacitance between tip and Au dot falls in the 10 Ϫ19 F range, implying that the origin of the monitored step structure can be interpreted to arise from single-electron tunnel effects ͑Coulomb staircase͒. As the nanometer Schottky contact forms a not well-insulating configuration, the development of negative differential resistance might be explained by a leaking of excess carriers from the dot to the immediate surroundings.

Research paper thumbnail of Integration of point-contact microscopy and atomic-force microscopy: Application to characterization of graphite/Pt(111)

Physical Review B, 1999

The electrical current through the point-contact junction of an AFM tip is used to image the surf... more The electrical current through the point-contact junction of an AFM tip is used to image the surfaces of bulk graphite ͑HOPG͒ and the surface of a graphitized carbon monolayer on Pt͑111͒ under ultra-high-vacuum ͑UHV͒ conditions. Lattice-resolved images are obtained simultaneously in topography, lateral friction, and contact current channels. Lattice resolution in current maps persisted up to 0.9 mA and pressures of up to 5 GPa. In both bulk graphite and the case of graphitized carbon monolayer on Pt͑111͒, the current images show only one maximum per unit cell. In addition, the contact current images of the graphite monolayer reveal local conductivity variations. We observed local conductivity variations in the form of moiré superstructures resulting from high order commensurability with the Pt lattice. ͓S0163-1829͑99͒03248-8͔ PRB 60 ENACHESCU, SCHLEEF, OGLETREE, AND SALMERON * Permanent address:

Research paper thumbnail of Evolution of visible photoluminescence and surface morphology of ultrathin porous Si films imaged by scanning tunneling microscopy

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1994

Research paper thumbnail of Making, Breaking and Sliding of Nanometer-Scale Contacts

Research paper thumbnail of An electron microscopy study of wear in polysilicon microelectromechanical systems in ambient air

Thin Solid Films, 2007

Wear is a critical factor in determining the durability of microelectromechanical systems (MEMS).... more Wear is a critical factor in determining the durability of microelectromechanical systems (MEMS). While the reliability of polysilicon MEMS has received extensive attention, the mechanisms responsible for this failure mode at the microscale have yet to be conclusively determined. We have used on-chip polycrystalline silicon side-wall friction MEMS specimens to study active mechanisms during sliding wear in ambient air. Worn parts were examined by analytical scanning and transmission electron microscopy, while local temperature changes were monitored using advanced infrared microscopy. Observations show that small amorphous debris particles (∼50-100 nm) are removed by fracture through the silicon grains (∼500 nm) and are oxidized during this process. Agglomeration of such debris particles into larger clusters also occurs. Some of these debris particles/clusters create plowing tracks on the beam surface. A nano-crystalline surface layer (∼20-200 nm), with higher oxygen content, forms during wear at and below regions of the worn surface; its formation is likely aided by high local stresses. No evidence of dislocation plasticity or of extreme local temperature increases was found, ruling out the possibility of high temperature-assisted wear mechanisms.

Research paper thumbnail of New laser ablation chamber for producing carbon nanomaterials using excimer laser

Materials Research Innovations, 2015

ABSTRACT Carbon nanomaterials have been obtained using an innovative laser ablation chamber desig... more ABSTRACT Carbon nanomaterials have been obtained using an innovative laser ablation chamber design. The chamber has several improvements of the design including a longer oven and a longer cold finger as compared to other designs. The laser employed in these experiments has been a KrF excimer laser. The chamber has been successfully used for obtaining nano-onions and single walled carbon nanotubes by laser ablation. A novel target preparation method has been developed and employed to incorporate metal catalysts without the need of pressing or hot pressing of the targets. The ablation products, obtained using this new chamber design, have been investigated by transmission electron microscopy and micro-Raman spectroscopy, as well as thermogravimetric analysis measurements.

Research paper thumbnail of Synthesis of single-wall carbon nanotubes by excimer laser ablation

Surface Engineering and Applied Electrochemistry, 2014

ABSTRACT We report the KrF excimer laser ablation of carbonaceous targets in an innovative laser ... more ABSTRACT We report the KrF excimer laser ablation of carbonaceous targets in an innovative laser ablation chamber. The targets have been prepared using a new approach, without pressing or hot pressing of the composition. The Co/Ni doped target has yielded single-wall carbon nanotubes with a narrow diameter distribution. High-resolution transmission electron microscopy has been used along with the confocal Raman microscopy to characterize the products obtained. Thermogravimetric analysis confirms the presence of multiple carbonaceous species with different oxidation temperatures.

Research paper thumbnail of Coulomb staircase with negative differential resistance at room temperature for a metal tip-metal dot-semiconductor double junction

Surface Science, 1996

Nanometer-sized Au dots have been fabricated on p-type Si(lll) substrates by field-induced transf... more Nanometer-sized Au dots have been fabricated on p-type Si(lll) substrates by field-induced transfer of tip material in a scanning tunneling microscope (STM). Positioning the STM tip on a very small dot ( 10-15 nm in diameter) allows observation of equidistant steps with a spacing of several 100 mV and regions with negative differential resistance (NDR) in the current-voltage characteristic. Numerical simulations confirm that the capacitance between the tip and the Au dot falls into the 10 -19 F range. The occurrence of NDR might be explained by a leaking of excess caxriers from the dot to the immediate $urrotnldings.

Research paper thumbnail of Sensing Dipole Fields at Atomic Steps with Combined Scanning Tunneling and Force Microscopy

Physical Review Letters, 2005

The electric field of dipoles localized at the atomic steps of metal surfaces due to the Smolucho... more The electric field of dipoles localized at the atomic steps of metal surfaces due to the Smoluchowski effect were measured from the electrostatic force exerted on the biased tip of a scanning tunneling microscope. By varying the tip-sample bias the contribution of the step dipole was separated from changes in the force due to van der Waals and polarization forces. Combined with electrostatic calculations, the method was used to determine the local dipole moment in steps of different heights on Au(111) and on the twofold surface of an Al-Ni-Co decagonal quasicrystal.

Research paper thumbnail of Imaging and manipulation properties of nanoparticles in scanning tunneling microscopy

Nanotechnology, 1996

Nanometer-sized particles manufactured by two different techniques and deposited on wet chemicall... more Nanometer-sized particles manufactured by two different techniques and deposited on wet chemically treated Si(111) substrates are explored at room temperature with a scanning tunneling microscope (STM) in the light of their stability upon repeated imaging and the feasibility to manipulate them with the probing tip. The two preparation techniques include field-induced transfer of Au from the STM tip stimulated by voltage pulsing and a wet chemical method. In the latter case, a colloidal solution of specified admixtures is prepared delivering CdS particles with a diameter of approximately 5 nm. In STM images, they protrude as 50-nm-high isolated hillocks or as one particle-high islands containing a varying number of individual constituents. The STM imaging process does not cause any changes in the structure of the islands or in the position of single particles. Upon traversing the tip through particle islands, several constituents can be removed. Depending on the deposition conditions in the field-induced transfer mode, the created Au deposits either adhere strongly to the substrate or they are displaced by the STM tip while imaging.

Research paper thumbnail of Electron-stimulated desorption of hydrogen from the Si(111) surface by scanning tunneling microscopy

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1996

Preparation of suitable silicon (111) wafers in weakly alkaline HF solutions results in the forma... more Preparation of suitable silicon (111) wafers in weakly alkaline HF solutions results in the formation of atomically flat, hydrogen-terminated surfaces. Under high-vacuum conditions, the scanning tunneling microscope has been employed to selectively desorb the passivating hydrogen from nanometer-sized surface regions. The hydrogen depassivation process is studied as a function of current and applied bias voltage, voltage polarity, and exposure time to incident electrons either on individual surface locations or by varying the speed of tip motion to control the electron dose. The experimental findings are interpreted in terms of two distinct desorption mechanisms and the respective desorption yields are specified. {copyright} {ital 1996 American Vacuum Society}

Research paper thumbnail of The role of contaminants in the variation of adhesion, friction, and electrical conduction properties of carbide-coated scanning probe tips and Pt(111) in ultrahigh vacuum

Journal of Applied Physics, 2004

Scanning probe microscopy was used to investigate the tribological properties of nanoscale tips i... more Scanning probe microscopy was used to investigate the tribological properties of nanoscale tips in contact with a Pt͑111͒ single-crystal surface under ultrahigh vacuum conditions. The tips were coated with a tungsten carbide film, which contained a significant fraction of oxygen. The electrically conductive tip made it possible to alternate between contact measurements and noncontact scanning tunneling microscopy. Several types of interfaces were found depending on the chemical state of the surfaces. The first type is characterized by strong irreversible adhesion followed by material transfer between tip and sample. Low adhesion and no material transfer characterize a second type of contact, which are associated with the presence of passivating adsorbates in both ͑full passivation͒ or in one of the two contacting surfaces ͑half-passivation͒. Half-passivated contacts in which the clean side is the Pt͑111͒ sample gave rise to periodic stick-slip friction behavior with a period equal to the atomic lattice constant of the Pt͑111͒ surface. Local electrical conductivity measurements show a clear correlation between electronic and friction properties, with ohmic behavior on clean regions of the Pt surface and semiconductor-like behavior on areas covered with adsorbates.

Research paper thumbnail of Stable nanostructuring of ultrathin porous silicon films by scanning tunneling microscopy

Journal of Applied Physics, 1996

The capability of producing electronically induced modifications in ultrathin ͑ϳ20 nm͒ light-emit... more The capability of producing electronically induced modifications in ultrathin ͑ϳ20 nm͒ light-emitting porous silicon ͑PS͒ films by use of a scanning tunneling microscope ͑STM͒ operated in a high-vacuum environment is demonstrated. Upon increasing the tunnel current to 2 nA and the tunnel voltage beyond a threshold value of ϳ7 V, structures 20-50 nm in width can be created to any desired pattern. These nanopatterns are stable at least for four days at room temperature. Experiments with both voltage polarities but equal power densities reveal that these structures can only be induced by directing the intense electron beam provided by the STM tip towards the sample surface, excluding pure thermal effects for the layer modifying process. These observations can be well explained by a model which includes a local increase in the density of defect states in deep-layer regions of the PS layer, which might be accompanied by a local quenching of the photoor electroluminescence activity.

Research paper thumbnail of C–Ni amorphous multilayers studied by atomic force microscopy

Applied Surface Science, 2000

Amorphous C-Ni superlattice films designed as normal-incidence reflector for 5 nm have been grown... more Amorphous C-Ni superlattice films designed as normal-incidence reflector for 5 nm have been grown on quartz substrate Ž . by magnetron sputter deposition. A new digital signal processor-based atomic force microscope AFM system has been involved in investigating the surface topography of the final surface of the multilayer structure as well as the substrate. In the first step, roughness measurements involving AFM-tips with different radius curvature indicate the C-Ni multilayer surface profile as being in the nanometer and sub-nanometer range, representing a high quality ended surface of the reflectors. In the Ž . second step, a study of the roughness distribution using power spectral density functions PSD calculated from AFM measurements was performed. The analysis of the amplitudes of the PSD spectra for the multilayer and the substrate surface provided the evidence of the optimized deposition parameters. The shape of the PSD curves for multilayer and substrate reveals fractal profiles with the same distribution of the roughness for both of them, attesting the influence of the substrate on the quality of the reflectors. q

Research paper thumbnail of Correlation of surface morphology with luminescence of porous Si films by scanning tunneling microscopy

Applied Physics Letters, 1994

The evolution of visible photoluminescence is demonstrated by measuring the surface morphology of... more The evolution of visible photoluminescence is demonstrated by measuring the surface morphology of thin (-20 nm) electrochemically etched porous silicon (PS) films with scanning tunneling microscopy (STM). Using low current densities, three sorts of samples were prepared under different conditions: In the dark (A), under illumination with ultraviolet (UV) light (B), and in the dark followed by a postphotochemical treatment (C). Upon UV light excitation, type A samples do not emit visible light, while samples of type B and C show weak and efficient photoluminescence in the visible range, respectively. STM imaging of these PS layers reveals a considerable decrease in the lateral dimensions of the surface features from approximately 10 nm (type A) to roughly 2 nm (type C), in accordance with the quantum confinement approach in describing the luminescence properties.

Research paper thumbnail of Writing electronic nanometer structures into porous Si films by scanning tunneling microscopy

Applied Physics Letters, 1994

Under conditions of increased tunnel current and voltage, with the electron flow directed towards... more Under conditions of increased tunnel current and voltage, with the electron flow directed towards the sample, nanometer-scale structures have been written into ultrathin (-20 nm) light-emitting porous silicon (PS) films using scanning tunneling microscopy in a high-vacuum environment. For the writing process, a threshold voltage of -4.5 V.is observed and the resulting dimensions range between 20 and 50 nm. Depending on the writing parameters, the modified regions relax or remain stable during the observation time of several days at room temperature. These results can be assigned, in the first case, to a disruption of a small number of bonds, followed by reconfiguration, and a charging of dangling bond sites, followed by carrier release, in near surface regions. In the latter case, the creation of time-stable defect states within the PS layer is proposed.

Research paper thumbnail of Observation of proportionality between friction and contact area at the nanometer scale

Tribology Letters, 1999

The nanotribological properties of a hydrogen-terminated diamond(111)/tungsten-carbide interface ... more The nanotribological properties of a hydrogen-terminated diamond(111)/tungsten-carbide interface have been studied using ultra-high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten-carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer-scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin-Müller-Toporov continuum mechanics model.

Research paper thumbnail of Atomic Force Microscopy Study of an Ideally Hard Contact: The Diamond(111)/Tungsten Carbide Interface

Physical Review Letters, 1998

Research paper thumbnail of Observation of Coulomb staircase and negative differential resistance at room temperature by scanning tunneling microscopy

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1996

Nanometer-sized Au dots have been fabricated on p-type Si͑111͒ substrates by field-induced transf... more Nanometer-sized Au dots have been fabricated on p-type Si͑111͒ substrates by field-induced transfer of tip material in a scanning tunneling microscope, thus forming a nanometer Schottky contact. Positioning the scanning tunneling microscope tip on a very tiny dot ͑10-15 nm in diameter͒, equidistant steps with a spacing of several 100 mV and regions with negative differential resistance are observed in the current-voltage characteristic. Numerical simulations confirm that the determinant capacitance between tip and Au dot falls in the 10 Ϫ19 F range, implying that the origin of the monitored step structure can be interpreted to arise from single-electron tunnel effects ͑Coulomb staircase͒. As the nanometer Schottky contact forms a not well-insulating configuration, the development of negative differential resistance might be explained by a leaking of excess carriers from the dot to the immediate surroundings.

Research paper thumbnail of Integration of point-contact microscopy and atomic-force microscopy: Application to characterization of graphite/Pt(111)

Physical Review B, 1999

The electrical current through the point-contact junction of an AFM tip is used to image the surf... more The electrical current through the point-contact junction of an AFM tip is used to image the surfaces of bulk graphite ͑HOPG͒ and the surface of a graphitized carbon monolayer on Pt͑111͒ under ultra-high-vacuum ͑UHV͒ conditions. Lattice-resolved images are obtained simultaneously in topography, lateral friction, and contact current channels. Lattice resolution in current maps persisted up to 0.9 mA and pressures of up to 5 GPa. In both bulk graphite and the case of graphitized carbon monolayer on Pt͑111͒, the current images show only one maximum per unit cell. In addition, the contact current images of the graphite monolayer reveal local conductivity variations. We observed local conductivity variations in the form of moiré superstructures resulting from high order commensurability with the Pt lattice. ͓S0163-1829͑99͒03248-8͔ PRB 60 ENACHESCU, SCHLEEF, OGLETREE, AND SALMERON * Permanent address:

Research paper thumbnail of Evolution of visible photoluminescence and surface morphology of ultrathin porous Si films imaged by scanning tunneling microscopy

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1994

Research paper thumbnail of Making, Breaking and Sliding of Nanometer-Scale Contacts

Research paper thumbnail of An electron microscopy study of wear in polysilicon microelectromechanical systems in ambient air

Thin Solid Films, 2007

Wear is a critical factor in determining the durability of microelectromechanical systems (MEMS).... more Wear is a critical factor in determining the durability of microelectromechanical systems (MEMS). While the reliability of polysilicon MEMS has received extensive attention, the mechanisms responsible for this failure mode at the microscale have yet to be conclusively determined. We have used on-chip polycrystalline silicon side-wall friction MEMS specimens to study active mechanisms during sliding wear in ambient air. Worn parts were examined by analytical scanning and transmission electron microscopy, while local temperature changes were monitored using advanced infrared microscopy. Observations show that small amorphous debris particles (∼50-100 nm) are removed by fracture through the silicon grains (∼500 nm) and are oxidized during this process. Agglomeration of such debris particles into larger clusters also occurs. Some of these debris particles/clusters create plowing tracks on the beam surface. A nano-crystalline surface layer (∼20-200 nm), with higher oxygen content, forms during wear at and below regions of the worn surface; its formation is likely aided by high local stresses. No evidence of dislocation plasticity or of extreme local temperature increases was found, ruling out the possibility of high temperature-assisted wear mechanisms.

Research paper thumbnail of New laser ablation chamber for producing carbon nanomaterials using excimer laser

Materials Research Innovations, 2015

ABSTRACT Carbon nanomaterials have been obtained using an innovative laser ablation chamber desig... more ABSTRACT Carbon nanomaterials have been obtained using an innovative laser ablation chamber design. The chamber has several improvements of the design including a longer oven and a longer cold finger as compared to other designs. The laser employed in these experiments has been a KrF excimer laser. The chamber has been successfully used for obtaining nano-onions and single walled carbon nanotubes by laser ablation. A novel target preparation method has been developed and employed to incorporate metal catalysts without the need of pressing or hot pressing of the targets. The ablation products, obtained using this new chamber design, have been investigated by transmission electron microscopy and micro-Raman spectroscopy, as well as thermogravimetric analysis measurements.

Research paper thumbnail of Synthesis of single-wall carbon nanotubes by excimer laser ablation

Surface Engineering and Applied Electrochemistry, 2014

ABSTRACT We report the KrF excimer laser ablation of carbonaceous targets in an innovative laser ... more ABSTRACT We report the KrF excimer laser ablation of carbonaceous targets in an innovative laser ablation chamber. The targets have been prepared using a new approach, without pressing or hot pressing of the composition. The Co/Ni doped target has yielded single-wall carbon nanotubes with a narrow diameter distribution. High-resolution transmission electron microscopy has been used along with the confocal Raman microscopy to characterize the products obtained. Thermogravimetric analysis confirms the presence of multiple carbonaceous species with different oxidation temperatures.

Research paper thumbnail of Coulomb staircase with negative differential resistance at room temperature for a metal tip-metal dot-semiconductor double junction

Surface Science, 1996

Nanometer-sized Au dots have been fabricated on p-type Si(lll) substrates by field-induced transf... more Nanometer-sized Au dots have been fabricated on p-type Si(lll) substrates by field-induced transfer of tip material in a scanning tunneling microscope (STM). Positioning the STM tip on a very small dot ( 10-15 nm in diameter) allows observation of equidistant steps with a spacing of several 100 mV and regions with negative differential resistance (NDR) in the current-voltage characteristic. Numerical simulations confirm that the capacitance between the tip and the Au dot falls into the 10 -19 F range. The occurrence of NDR might be explained by a leaking of excess caxriers from the dot to the immediate $urrotnldings.

Research paper thumbnail of Sensing Dipole Fields at Atomic Steps with Combined Scanning Tunneling and Force Microscopy

Physical Review Letters, 2005

The electric field of dipoles localized at the atomic steps of metal surfaces due to the Smolucho... more The electric field of dipoles localized at the atomic steps of metal surfaces due to the Smoluchowski effect were measured from the electrostatic force exerted on the biased tip of a scanning tunneling microscope. By varying the tip-sample bias the contribution of the step dipole was separated from changes in the force due to van der Waals and polarization forces. Combined with electrostatic calculations, the method was used to determine the local dipole moment in steps of different heights on Au(111) and on the twofold surface of an Al-Ni-Co decagonal quasicrystal.

Research paper thumbnail of Imaging and manipulation properties of nanoparticles in scanning tunneling microscopy

Nanotechnology, 1996

Nanometer-sized particles manufactured by two different techniques and deposited on wet chemicall... more Nanometer-sized particles manufactured by two different techniques and deposited on wet chemically treated Si(111) substrates are explored at room temperature with a scanning tunneling microscope (STM) in the light of their stability upon repeated imaging and the feasibility to manipulate them with the probing tip. The two preparation techniques include field-induced transfer of Au from the STM tip stimulated by voltage pulsing and a wet chemical method. In the latter case, a colloidal solution of specified admixtures is prepared delivering CdS particles with a diameter of approximately 5 nm. In STM images, they protrude as 50-nm-high isolated hillocks or as one particle-high islands containing a varying number of individual constituents. The STM imaging process does not cause any changes in the structure of the islands or in the position of single particles. Upon traversing the tip through particle islands, several constituents can be removed. Depending on the deposition conditions in the field-induced transfer mode, the created Au deposits either adhere strongly to the substrate or they are displaced by the STM tip while imaging.

Research paper thumbnail of Electron-stimulated desorption of hydrogen from the Si(111) surface by scanning tunneling microscopy

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1996

Preparation of suitable silicon (111) wafers in weakly alkaline HF solutions results in the forma... more Preparation of suitable silicon (111) wafers in weakly alkaline HF solutions results in the formation of atomically flat, hydrogen-terminated surfaces. Under high-vacuum conditions, the scanning tunneling microscope has been employed to selectively desorb the passivating hydrogen from nanometer-sized surface regions. The hydrogen depassivation process is studied as a function of current and applied bias voltage, voltage polarity, and exposure time to incident electrons either on individual surface locations or by varying the speed of tip motion to control the electron dose. The experimental findings are interpreted in terms of two distinct desorption mechanisms and the respective desorption yields are specified. {copyright} {ital 1996 American Vacuum Society}

Research paper thumbnail of The role of contaminants in the variation of adhesion, friction, and electrical conduction properties of carbide-coated scanning probe tips and Pt(111) in ultrahigh vacuum

Journal of Applied Physics, 2004

Scanning probe microscopy was used to investigate the tribological properties of nanoscale tips i... more Scanning probe microscopy was used to investigate the tribological properties of nanoscale tips in contact with a Pt͑111͒ single-crystal surface under ultrahigh vacuum conditions. The tips were coated with a tungsten carbide film, which contained a significant fraction of oxygen. The electrically conductive tip made it possible to alternate between contact measurements and noncontact scanning tunneling microscopy. Several types of interfaces were found depending on the chemical state of the surfaces. The first type is characterized by strong irreversible adhesion followed by material transfer between tip and sample. Low adhesion and no material transfer characterize a second type of contact, which are associated with the presence of passivating adsorbates in both ͑full passivation͒ or in one of the two contacting surfaces ͑half-passivation͒. Half-passivated contacts in which the clean side is the Pt͑111͒ sample gave rise to periodic stick-slip friction behavior with a period equal to the atomic lattice constant of the Pt͑111͒ surface. Local electrical conductivity measurements show a clear correlation between electronic and friction properties, with ohmic behavior on clean regions of the Pt surface and semiconductor-like behavior on areas covered with adsorbates.

Research paper thumbnail of Stable nanostructuring of ultrathin porous silicon films by scanning tunneling microscopy

Journal of Applied Physics, 1996

The capability of producing electronically induced modifications in ultrathin ͑ϳ20 nm͒ light-emit... more The capability of producing electronically induced modifications in ultrathin ͑ϳ20 nm͒ light-emitting porous silicon ͑PS͒ films by use of a scanning tunneling microscope ͑STM͒ operated in a high-vacuum environment is demonstrated. Upon increasing the tunnel current to 2 nA and the tunnel voltage beyond a threshold value of ϳ7 V, structures 20-50 nm in width can be created to any desired pattern. These nanopatterns are stable at least for four days at room temperature. Experiments with both voltage polarities but equal power densities reveal that these structures can only be induced by directing the intense electron beam provided by the STM tip towards the sample surface, excluding pure thermal effects for the layer modifying process. These observations can be well explained by a model which includes a local increase in the density of defect states in deep-layer regions of the PS layer, which might be accompanied by a local quenching of the photoor electroluminescence activity.

Research paper thumbnail of C–Ni amorphous multilayers studied by atomic force microscopy

Applied Surface Science, 2000

Amorphous C-Ni superlattice films designed as normal-incidence reflector for 5 nm have been grown... more Amorphous C-Ni superlattice films designed as normal-incidence reflector for 5 nm have been grown on quartz substrate Ž . by magnetron sputter deposition. A new digital signal processor-based atomic force microscope AFM system has been involved in investigating the surface topography of the final surface of the multilayer structure as well as the substrate. In the first step, roughness measurements involving AFM-tips with different radius curvature indicate the C-Ni multilayer surface profile as being in the nanometer and sub-nanometer range, representing a high quality ended surface of the reflectors. In the Ž . second step, a study of the roughness distribution using power spectral density functions PSD calculated from AFM measurements was performed. The analysis of the amplitudes of the PSD spectra for the multilayer and the substrate surface provided the evidence of the optimized deposition parameters. The shape of the PSD curves for multilayer and substrate reveals fractal profiles with the same distribution of the roughness for both of them, attesting the influence of the substrate on the quality of the reflectors. q

Research paper thumbnail of Correlation of surface morphology with luminescence of porous Si films by scanning tunneling microscopy

Applied Physics Letters, 1994

The evolution of visible photoluminescence is demonstrated by measuring the surface morphology of... more The evolution of visible photoluminescence is demonstrated by measuring the surface morphology of thin (-20 nm) electrochemically etched porous silicon (PS) films with scanning tunneling microscopy (STM). Using low current densities, three sorts of samples were prepared under different conditions: In the dark (A), under illumination with ultraviolet (UV) light (B), and in the dark followed by a postphotochemical treatment (C). Upon UV light excitation, type A samples do not emit visible light, while samples of type B and C show weak and efficient photoluminescence in the visible range, respectively. STM imaging of these PS layers reveals a considerable decrease in the lateral dimensions of the surface features from approximately 10 nm (type A) to roughly 2 nm (type C), in accordance with the quantum confinement approach in describing the luminescence properties.

Research paper thumbnail of Writing electronic nanometer structures into porous Si films by scanning tunneling microscopy

Applied Physics Letters, 1994

Under conditions of increased tunnel current and voltage, with the electron flow directed towards... more Under conditions of increased tunnel current and voltage, with the electron flow directed towards the sample, nanometer-scale structures have been written into ultrathin (-20 nm) light-emitting porous silicon (PS) films using scanning tunneling microscopy in a high-vacuum environment. For the writing process, a threshold voltage of -4.5 V.is observed and the resulting dimensions range between 20 and 50 nm. Depending on the writing parameters, the modified regions relax or remain stable during the observation time of several days at room temperature. These results can be assigned, in the first case, to a disruption of a small number of bonds, followed by reconfiguration, and a charging of dangling bond sites, followed by carrier release, in near surface regions. In the latter case, the creation of time-stable defect states within the PS layer is proposed.

Research paper thumbnail of Observation of proportionality between friction and contact area at the nanometer scale

Tribology Letters, 1999

The nanotribological properties of a hydrogen-terminated diamond(111)/tungsten-carbide interface ... more The nanotribological properties of a hydrogen-terminated diamond(111)/tungsten-carbide interface have been studied using ultra-high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten-carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer-scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin-Müller-Toporov continuum mechanics model.

Research paper thumbnail of Atomic Force Microscopy Study of an Ideally Hard Contact: The Diamond(111)/Tungsten Carbide Interface

Physical Review Letters, 1998