Techniken der Oberflächenphysik (Technique of Surface Physics)

Techniken der Oberflächenphysik (Technique of Surface Physics) Yong Lei & Fabian Grote Fachgebiet 3D-Nanostrukturierung, Institut für Physik Contact: yong.lei@tu-ilmenau.de; fabian.grote@tu-ilmenau.de Office: Heliosbau 1102, Prof. Schmidt-Straße 26 (tel: 3748) www.tu-ilmenau.de/nanostruk Vorlesung: Mittwochs (U), 9 10:30, C 108 Übung: Mittwochs (G), 9 10:30, C 108

Surface Physics - Why? Objects are contacted via their surface. Chemical reactions: Catalysis, electrodes of batteries Friction and Lubrication Nanotechnology is Surface Physics

Main contents of this course Surfaces become more important for smaller objects Almost all aspects of physical properties are related to their surfaces (nano-surfaces): Optical properties (band-gap, defect emissions) Sensing properties (gas, chemical and bio-sensors) Field-emission properties Devices (super-capacitors, sensors, optical )

Class 1 (an introduction) A general introduction of fundamentals of surface physics and their most important points (how to fabricate the surfaces especially within nano-sized range) (how to characterize the surfaces) (what the main properties of surfaces) (what s the main applications of surfaces)

Nobel Prizes with research related to surface physics and structures: Kai M. Siegbahn (Swedish) Nobel Prize 1981 Physics Developing the method of Electron Spectroscopy for Chemical Analysis, now usually described as X-ray photoelectron spectroscopy (XPS) G. Binnig (German) & H. Rohrer (Swiss) Nobel Prize 1986 Physics Designing of the scanning tunneling microscope (STM) SPM systems

Gerhard Ertl (German) Nobel Prize 2007 Chemistry for his studies of chemical processes on solid surfaces Albert Fert (French) & Peter Grünberg (German) Nobel Prize 2007 Physics Interfaces - Giant magnetoresistance effect (GMR) which is a breakthrough in gigabyte hard disk drives.

Konstantin Novoselov & Andre Geim (Russian) Nobel Prize 2010 Physics for groundbreaking experiments regarding the two-dimensional graphene 7

1996: Curl, Kroto, Smalley 1985 or1986: fullerenes (C60, bucky balls); 2010: Geim, Novoselov 2005-2007: 2D graphene The allotropes of carbon: hardest naturally occurring substance, diamond one of the softest known substances, graphite. For carbon nanotubes CNT (by Ijima in 1991) and the equally important discovery of inorganic fullerene structures (by Tenne) Allotropes of carbon: a) diamond; b) graphite; c) lonsdaleite; d f) fullerenes (C 60, C 540, C 70 ); g) amorphous carbon; h) carbon nanotube. from http://en.wikipedia.org/wiki/carbon.

Most important structural aspects of nanostructures: Surface Extremely large surface area (very large surface/volume ratio): when the dimensions decrease from micron level to nano level, the surface area increases by 3 orders in magnitude. This will lead to much improved and enhanced physical properties (sensing, optical, catalysis...): Cube Cubic structures divided into 8 pieces surface area 2 times(doubled) Cube Cubic structures divided into 1000 pieces surface area 10 times

Surface charge properties of structures are the major point of functions of sensing devices. The main reason of the high interest in the use of nanostructures is the large surface-to-volume ratio, so that more surface atoms to participate in the surface reactions The electronic, chemical, and optical processes on metal oxides concerning the sensing, which is benefit from reduction in size to the nano range (Kolmakov, Annu Rev Mater Res 2004)

Surface plasmon resonance: plasmons propagate in x- and y-directions along metal-dielectric interface (distances ~ tens to hundreds microns), and decay in z- direction. The interaction between surface-confined EM wave and surface layer leads to shifts in the plasmon resonance condition. Localized surface plasmons: light interacts with particles much smaller size than incident light wavelength. This leads to a plasmon that oscillates around nanoparticles with a LSPR frequency. The shape, size, material, and local dielectric properties all of which determine the LSPR wavelength. Schematic of (a) a surface plasmon resonance and (b) a localized surface plasmon resonance. (Katherine A., Annu. Rev. Phys. Chem. 2007. 58, 267.)

Characterization of surfaces An appropriate characterization will play a crucial role in determining various surface structures and their properties (especially for nanosurfaces. Three broadly approved aspects of characterization are 1. Morphology 2. Crystalline structure 3. Chemical analysis

SEM: Scanning Electron Microscope; STM/AFM: Scanning Tunneling Microscope/Atomic Force Microscope TEM: Analytical Transmission Electron Microscope X-Ray: X-ray Morphology; IP: Image Processing; LM: Lightweight Morphology; RBS: Rutherford Backscattering Spectrometry (Kelsall et al., Nanoscale science and technology. 2005)

TEM: Analytical Transmission Electron Microscopy; AES: Auger Electron Spectrometer; XRD: X-ray Diffraction; RBS: Rutherford Backscattering Spectrometry; XPS: X-ray Photoelectron Spectrometer; (Kelsall et al., Nanoscale science and technology. 2005)

SEM: Scanning Electron Microscopy; ATEM: Analytical Transmission Electron Microscopy; AEM: Auger Electron Microscopy. XRD: X-ray Diffraction; LEED: Lowenergy electron diffraction; RBS: Rutherford Backscattering Spectrometry (Kelsall et al., Nanoscale science and technology. 2005)

Surface patterns in nature Structural color function of surface patterns butterfly 1 µm peacock packing of melanin cylinders (provided by L Chi)

Surface patterns and structures (artificial) and their applications in diverse (micro-electronic) devices Dual-core CPU feature-size 45 nm From Intel Homepage, Public Relations

Surface Nano-Patterning Fabrication of surface nanostructures Memory devices with high integration density; Field emission devices; Sensors with high sensitivity; Optical devices with tunable properties What is an excellent surface nano-patterning technique? 1. Ability to prepare surface patterns within the nanosized range; 2. Well-defined surface nano-patterns; 3. Large pattern area high throughput; 4. A general process applicable; 5. Low cost. Perfect?

Some surface nano-patterning techniques in fabricating ordered surface nanostructures Electron-beam lithography Excellent structural controlling Low throughput High equipment costs Imprint technologies High throughput Wear Structures with low aspect ratio Self assembly Low costs High throughput Limited class of materials Low structural controlling Alternative method that combines these advantages and is applicable for a broad range of surface nanostructures? UTAM (ultra-thin alumina mask) surface nano-patterning: Template-based surface nano-fabrications

Porous Alumina Membranes (PAMs) Interesting and useful features: highly ordered pore arrays + large area Nanometer-sized pores High aspect ratio size controllable (10 400 nm) Configuration diagram of the PAMs

Porous Alumina Membranes (PAMs) (a) (b) Regular arrays of short (a) and long Ni nanowires (b) after the removal of PAM, the diameter is about 90 nm, the length is about 800-1000 nm (a) and 3-4 μm (b), respectively. thus the aspect ratio of the nanowires are about 10 (a) and 40 (b), respectively.

UTAM surface nano-patterning technique Fabrication of Highly Ordered Nanoparticle Arrays Using Ultra-thin Alumina Mask (UTAM) Motivation Use ultra-thin ordered porous alumina as evaporation or etching masks, and transfer the regularity of the pore arrays to the nanostructure arrays on substrates.

Fabrication process Fabricating ultra-thin alumina masks (UTAM) on Al foils and then mounting them onto the surface of silicon wafers First alumina layer Al Si foil wafer Ultra-thin Second alumina layer mask

Fabrication of the nanodot arrays Si wafer Ultra-thin Nanoparticle alumina array mask

CdS replicated mask Alumina CdS nanodots Highly ordered CdS nanodot arrays, UTAMs and CdS top layer on the surface of the UTAM.

Nanodots (top view, Pd) Nanoholes (top view, Si)

Tuning of the shapes and sizes of UTAM-prepared nanostructures To control the structural parameters (shape, size and spacing) is very important Controllable sizes and shapes: The pore diameters of the UTAMs can be adjusted from about 10 to 400 nm to yield nanoparticles of corresponding size. Nanometer-sized discs, hemispheres, hemi-ellipsoids, and conics (by changing the aspect ratio of the pores of the UTAMs, and the amount of material deposited through the UTAMs).

Highly ordered nano-disc arrays Highly ordered nano-disc arrays. Pore diameter, cell size and thickness of the UTAM are about 80, 105, and 160 nm, respectively. The aspect ratio of the apertures of the UTAM is about 1:2. The average height and size of the nano-discs are approximately 1.5 and 80 nm, respectively.

AFM Section Analysis of the nano-discs, the average height and size of the nano-discs are approximately 1.5 and 80 nm, respectively.

Highly ordered nano-hemisphere arrays Highly ordered nano-hemisphere arrays. Pore diameter, cell size and thickness of the UTAM are about 80, 105, and 240 nm, respectively. The aspect ratio of the apertures of the UTAM is about 1:3. The average height and base diameter of the nano-hemispheres are approximately 35-40 and 75 nm, respectively.

AFM Section Analysis of the nano-hemisphere. To accurately reflect the shape of the nanoparticles, we used the same dimension scale for the horizontal and vertical coordinates. The average height and base diameter of the nanohemispheres are approximately 35-40 and 75 nm, respectively.

Highly ordered nano-hemiellipsoid arrays Ordered nano-hemiellipsoid arrays. Pore diameter, cell size and thickness of the UTAM are about 80, 105, and 310 nm, respectively. The aspect ratio of the apertures of the UTAM is about 1:4. The average height and base diameter of the nano-hemiellipsoids are approximately 50-55 and 65 nm, respectively.

AFM Section Analysis of the nano-hemiellipsoids. To accurately reflect the shape of the nanoparticles, we used the same dimension scale for the horizontal and vertical coordinates. The average height and base diameter of the nanohemiellipsoids are approximately 50-55 and 65 nm, respectively.

Highly ordered nano-conic arrays Ordered nano-conic arrays. Pore diameter, cell size and thickness of the UTAM used in the fabrication process are about 80, 105, and 650 nm, respectively. The aspect ratio of the apertures of the UTAM is about 1:8. The average height and base diameter of the nano-conics are approximately 55-60 and 60 nm, respectively.

AFM Section Analysis of the nano-conics. To accurately reflect the shape of the nanoparticles, we used the same dimension scale for the horizontal and vertical coordinates. The average height and base diameter of the nano-conics are approximately 55-60 and 60 nm, respectively.

Aspect ratio and deposition duration Closure-effect Shadowing-effect Schematic outline of the shape and size adjustment of nanoparticles by changing the aspect ratio of the apertures of the UTAMs and the amount of material deposited through the UTAMs. (Y Lei, et al., Chem. Mater., 17, 580, 2005.)

Attractive features of the UTAM surface nano-patterning: Large pattern area (> 1cm 2 ) and high throughput; high density of the surface nanostructures (10 10-10 12 cm -2 ); a general process to prepare different patterns (semiconductors, metals); well-defined nanostructures; low equipment costs.

Tunable properties of UTAM-prepared surface patterns based on the adjustment of the structural parameters 1. Ordered arrays of metal/semiconductor core-shell nanodots with tunable core-shell structures and optical properties. (Y Lei, et al., J. Am. Chem. Soc., 127, 1487, 2005). 2. Ordered CdS nanodot arrays with adjustable luminescence properties. (Y Lei, et al., Appl. Phys. Lett., 86, 103106, 2005). 3. Ordered arrayed metal oxide nanodots with the similar size, shape, crystalline structure and orientation. This work is a step towards the goal of achieving iso-nanoparticle arrays and full property tuning. (Y Lei, et.al., Nanotechnology, 16, 1892, 2005). 4. Large-scale ordered carbon nanotube (CNT) arrays initiated from highly ordered metallic arrays on silicon substrates. (Y Lei, et al., Chem. Mater., 16, 2757, 2004).

Device applications of UTAM-prepared surface patterns 1. Metal insulator semiconductor (MIS) memory device based on ordered Ge nanodot arrays. (Z Chen, Y Lei, et al., J. Cry. Grow., 268, 560, 2004). 2. Fe/Pt multi-layer nanodot arrays with interesting magnetic properties (J Ellrich, Y Lei, H Hahn, patent application, 2008). 3. Other possible device applications (Lei Y, et al., Adv. Eng. Mater., 9, 343, 2007).

A challenging technical point for UTAM technique to realize quantum-sized surface structures (below 10-20 nm) Minimum pore diameter of UTAMs is about 10 nm impossible to synthesize surface structures smaller than 10 nm; the arrangement regularity and monodispersity of the pores are poor when the pore diameter is smaller than 20 nm; Prevents the fabrication of surface structures within or close to the quantum-sized range (below 10-20 nm) using the UTAM patterning technique largely limits the investigation of the quantum confinement effect using the UTAM surface nano-patterning process.

Well-controlled pore-opening process to the barrier layer of UTAMs realizing pore-opening and surface nanostructures within the quantumsized range An alumina barrier layer between the pore bottom and the aluminum foil of asprepared PAMs. It has a hemispherical and scalloped geometry. Using acidic etching solutions, the barrier layer can be thinned and finally removed.

UTAMs used in the pore-opening process were prepared using 0.3 M modulated H 2 SO 4 solutions (glycol/water: 3:2) under 25 V at 4 o C, cell-size 60 nm, pore-diameter 20 nm, barrier layer thickness 20 nm Pore-opening process was carried out using a 5 wt% H 3 PO 4 solution at 30 o C Before the etching, UTAMs were covered by a protecting PMMA layer on the top so that the H 3 PO 4 solution only etch on the bottom surface. (a) (b) 18.76 [nm] 20.00 [nm] 200.00 nm 500.00 x 500.00 nm o min 1:1 Before etching in 5 wt% H 3 PO 4 solution (30 o C) 0.00 200.00 nm 500.00 x 500.00 nm 10 mins surface 0.00 After 8 min etching in 5 wt% H 3 PO 4 solution (30 o C)

(c) (d) 24.08 [nm] 46.16 [nm] 200.00 nm 500.00 x 500.00 nm 10min 1:1 0.00 After 15 min etching in 5wt% H 3 PO 4 solution (30 o C) The pore diameter is about 5 nm (e) 200.00 nm 500.00 x 500.00 nm 2-18mins edge 0.00 After 18 min etching in 5 wt% H 3 PO 4 solution (30 o C) The pore diameter is about 10 nm (f) 40.98 [nm] 54.12 [nm] 200.00 nm 500.00 x 500.00 nm 2-24mins 0.00 200.00 nm 500.00 x 500.00 nm h20 30mins 0.00 After 24 min etching in 5 wt% H 3 PO 4 solution (30 o C) The pore diameter is about 17 nm After 30 min etching in 5 wt% H 3 PO 4 solution (30 o C) The pore diameter is about 22 nm

UTAMs with pore-openings smaller than 20 nm can be used to fabricate ordered arrays of quantum dots Using an UTAM with pore-openings ~ 17 nm (24 min etching time), largescale (~ 2 cm 2 ) ordered arrays of Au quantum-dots were prepared on a Si wafer. Based on the histogram and its Gaussian fit curve of the measured diameters of the Au nanodots, the average diameter of the nanodots is about 17.02 nm. (a) (b)

UTAM surface nano-patterning Barrier layer Quantum dot array 5 nm 10 nm 17 nm Small 2010, 6 (5), 695-699.

Three-Dimensional Surface Nano-Patterning: Concepts, Challenges and Applications (motivations) Multifunctional surface nano-structures Dual-core CPU feature-size 45 nm From Intel Homepage, Public Relations

Three-Dimensional Surface Nano-Patterning: Concepts, Challenges and Applications (motivations) Multifunctional surface nano-structures One of the most attractive advantages of nanomaterials (extremely large surface area) is missing in the existing 2-D surface nano-patterns nanodots Only way to increase the device density is to decrease the pattern size nanorings Large contacting influence from the substrate very large signal noises degrades device performance An efficient evolution from 2-D to 3-D surface nano-patterning: Change from nanodots or nanorings to nanowires or nanotubes

Three-Dimensional Surface Nano-Patterning: Concepts, Challenges and Applications (motivations) Multifunctional surface nano-structures A much larger surface area nanowires Possible to increase the device density in the lateral direction Much lower contacting influence from the substrate nanotubes An efficient evolution from 2-D to 3-D surface nano-patterning: Change from nanodots or nanorings to nanowires or nanotubes

From 2D to 3D surface patterns using templates

From 2D to 3D surface patterns using templates Large-scale free-standing metallic nanowires for 3D surface patterns: (Left): top view of nanowire array of an area of about 775 μm 2. (Right): high regularity of nanowire arrays.

3D Surface Nano-Patterning: Addressing Addressing System for 3-D surface nano-patterns with nano-scale resolution nanowire 1A Schematic of the addressing system (only shows an array of 3 3)

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