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Fachgebiet 3D-Nanostrukturierung, Institut für Physik
Contact: [email protected]
Office: Heisenbergbau (Gebäude V) 202, Unterpörlitzer Straße 38 (tel: 3748)
www.tu-ilmenau.de/nanostruk
Vorlesung: Mittwochs (G), 9 – 10:30, C 108 Übung: Mittwochs (U), 9 – 10:30, C 108
Prof. Yong Lei & Dr. Yang Xu
Techniken der Oberflächenphysik (Techniques of Surface Physics)
• Objects are contacted via their surface. • Chemical reactions: Catalysis, electrodes of batteries • Many properties are related: Friction and Lubrication • Nanotechnology is Surface Physics
Surface Physics - Why?
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 the fundamentals of surface physics and
their most important points
(what are the main properties of surfaces)
(how to characterize surfaces)
(how to fabricate surfaces especially within nano-sized range)
(what’s the main applications of surfaces)
Kai M. Siegbahn (Swedish) Nobel Prize 1981 Physics Developing the method of Electron Spectroscopy for Chemical Analysis, 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
Nobel Prizes with researches related to surface physics and structures:
• 2. Manipulation of single atoms or single molecules
Constant current image (topography) of an atomic layer iron on W(001) with defects and atoms.
High performance STM image showing atomic resolution on Si(111) (7nm x 7nm)
cobalt sulfide "nanoflower" structure synthesized on a Au(111) surface (9nm x 9nm)
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.
10
Konstantin Novoselov & Andre Geim (Russian) Nobel Prize 2010 Physics for groundbreaking experiments regarding the two-dimensional graphene
For carbon nanotubes – CNT (by
Ijima in 1991) and the equally
important discovery of inorganic
fullerene structures (by Tenne)
1996: Curl, Kroto, Smalley
1985 or 1986: fullerenes (C60,
bucky balls);
2010: Geim, Novoselov 2005-2007: 2D graphene
The allotropes of carbon:
hardest natural substance,
diamond
one of the softest known
substances, graphite.
Allotropes of carbon: a) diamond; b) graphite;
c) lonsdaleite; d–f) fullerenes (C60, C540, C70); g)
amorphous carbon; h) carbon nanotube.
from http://en.wikipedia.org/wiki/Carbon.
Graphene is a 1-atom thickness sheet of sp2-bonded carbon atoms that are
densely packed in a honeycomb crystal lattice. Graphene is easily visualized as an
atomic-scale wire made of carbon atoms and their bonds. Graphite consists of
many graphene sheets stacked together.
(http://en.wikipedia.org/wiki/Graphene)
Zigzag carbon nanotube
could be either semiconducting
or metallic
Armchair carbon nanotube
all metallic
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)
Characterization of surfaces
An appropriate characterization will play a crucial role in determining
various surface structures and their properties (especially for nano-
surfaces).
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)
Cross-section BSE image, showing pore
opening, pore wall, and SnO2 layer. Al2O3
membrane and SnO2 show different contrast. Tin dioxide is brighter compared to Al2O3 because of higher z-value
Cross-section SE-SEM image, showing UTAM filled with SnO2. The present of two different materials can not be observed clearly. BSE detection proves the existence of 2 materials.
Secondary electrons (SE-SEM) & Back scattered electrons (BSE)
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: Low-
energy electron diffraction; RBS: Rutherford Backscattering
Spectrometry (Kelsall et al., Nanoscale science and technology. 2005)
Surface patterns in nature
Structural color – function of surface patterns
1 µm
butterfly
peacock
packing of melanin cylinders (provided by L Chi)
Surface patterns and structures (artificial)
and their applications in diverse (micro-electronic) devices
From Intel Homepage, Public Relations
Dual-core CPU
feature-size 45 nm
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 ?
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
Some surface nano-patterning techniques
in fabricating ordered surface nanostructures
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.
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.
UTAM surface nano-patterning technique
Fabrication of Highly Ordered Nanoparticle Arrays Using Ultra-thin
Alumina Mask (UTAM)
Fabricating ultra-thin alumina masks (UTAM) on Al foils and then
mounting them onto the surface of silicon wafers
Al foil
First alumina layer
Al foil Al foil
Second alumina layer
Al foil
Ultra-thin alumina mask
Si wafer
Ultra-thin alumina mask
Fabrication process
Fabrication of the nanodot arrays
Ultra-thin alumina mask
Si wafer
Ultra-thin alumina mask
Si wafer
Ultra-thin alumina mask Nanoparticle array
Si wafer Si wafer
Highly ordered CdS nanodot arrays, UTAMs and CdS top layer on the surface of the UTAM.
CdS replicated mask
Alumina
CdS nanodots
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. 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.
Highly ordered nano-disc arrays
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. 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.
Highly ordered nano-hemisphere arrays
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 nano-
hemispheres are approximately 35-40 and 75 nm, respectively.
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.
Highly ordered nano-hemiellipsoid arrays
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 nano-
hemiellipsoids are approximately 50-55 and 65 nm, respectively.
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.
Highly ordered nano-conic arrays
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.
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.)
Closure-effect Shadowing-effect
Aspect ratio and deposition duration
Attractive features of the UTAM surface nano-patterning:
Large pattern area (> 1cm2) and high throughput;
high density of the surface nanostructures (1010 - 1012 cm -2);
a general process to prepare different patterns (semiconductors,
metals);
well-defined nanostructures;
low equipment costs.
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 quantum-
sized range
An alumina barrier layer between the pore bottom and the aluminum foil of as-
prepared 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
H2SO4 solutions (glycol/water: 3:2) under 25 V at 4 oC,
cell-size 60 nm, pore-diameter 20 nm, barrier layer thickness 20 nm
Pore-opening process was carried out using a 5 wt% H3PO4 solution at 30 oC
Before the etching, UTAMs were covered by a protecting PMMA layer on the top
so that the H3PO4 solution only etch on the bottom surface.
Before etching in 5 wt% H3PO4 solution (30 oC)
18.76
[nm]
0.00200.00 nm 500.00 x 500.00 nm
o min 1:1
(a)
After 8 min etching in 5 wt% H3PO4 solution (30 oC)
20.00
[nm]
0.00200.00 nm 500.00 x 500.00 nm
10 mins surface
(b)
After 18 min etching in 5 wt% H3PO4 solution (30 oC)
The pore diameter is about 10 nm
46.16
[nm]
0.00200.00 nm 500.00 x 500.00 nm
2-18mins edge
(d)
After 15 min etching in 5wt% H3PO4 solution (30 oC)
The pore diameter is about 5 nm
24.08
[nm]
0.00200.00 nm 500.00 x 500.00 nm
10min 1:1
(c)
After 24 min etching in 5 wt% H3PO4 solution (30 oC)
The pore diameter is about 17 nm
40.98
[nm]
0.00200.00 nm 500.00 x 500.00 nm
2-24mins
(e)
After 30 min etching in 5 wt% H3PO4 solution (30 oC)
The pore diameter is about 22 nm
54.12
[nm]
0.00200.00 nm 500.00 x 500.00 nm
h20 30mins
(f)
UTAM surface nano-patterning
Y. Lei, et al., Progress in Materials Science, 52, 465, 2007.
Attractive features of the UTAM surface nano-patterning
Large pattern area (> 1cm2) and high throughput;
high density of the surface nanostructures (1010 - 1012 cm-2);
a general process to prepare different patterns (semiconductors, metals);
well-defined nanostructures;
low equipment costs.
Y. Lei, et.al., J. Am. Chem. Soc., 127, 1487, 2005; Chem. Mater., 17, 580, 2005; Chem. Mater.,
16, 2757, 2004; Appl. Phys. Lett., 86, 103106, 2005; Nanotechnology, 16, 1892, 2005.
Barrier layer 5 nm
10 nm 17 nm
Quantum dot array
Small 2010, 6 (5), 695-699.
UTAM surface nano-patterning
Sub-100-nm Nanoparticle Arrays with Perfect Ordering, Tunable and Uniform
Dimensions Fabricated by Combining Nanoimprinting with UTAM Technique
Zhan Z.B., Lei Y., et al., ACS Nano, 8 (4), 3862–3868, 2014.
Al-Haddad, Lei, et al., ACS Nano, 9(8):8584-91, 2015
Wafer-scale (4 in. ) UTAM transferring
A free-standing 4-in.
wafer-scale UTAM
with the residual Al
frame
Perfect mounting
without any twisting,
folding, cracking and
contamination
Fabrication of Ag Nanoshell Arrays
S. Yang, Y. Lei, et al., Adv. Funct. Mater., 2010, 20, 2527
Surface patterning using polystyrene (PS) sphere template
The diameter of PS spheres can be controlled within 200 nm - 4.5 μm
Three-Dimensional Surface Nano-Patterning
Multifunctional surface nano-structures
An efficient evolution from 2-D to 3-D surface nano-patterning:
Change from nanodots or nanorings to nanowires or nanotubes
One of the most attractive advantages of nano-
materials (extremely large surface area) is
missing in the existing 2-D surface nano-patterns
Large contacting influence from the substrate →
very large signal noises → degrades device
performance
Only way to increase the device density is to
decrease the pattern size
A much larger surface area
Much lower contacting influence from the
substrate
Possible to increase the device density in the
lateral direction
From Intel Homepage, Public Relations
Dual-core CPU
feature-size 45 nm
nanodots
nanorings
nanowires
nanotubes
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 μm2. (Right): high regularity of nanowire arrays.
Metallic nanotube arrays (by ALD)
Cover almost all inner
surface of AAO
Schematic of the addressing system (only shows an array of 3 × 3)
Addressing System for 3-D surface nanostructures
with nano-scale resolution
3D Surface Nano-Patterning: Addressing
nanowire ‘1A’
Addressing system
1. The short-range pore regularity of UTAMs or AAO templates
→ Template with large-scale perfect pore arrays (to mm2 or even to cm2)
3D Surface Nano-Patterning: nano-templates with large-scale
(up to 1 mm2) perfect pore arrays without defects
2. Hexagonal pore arrangement
→ Rectangular pore arrangement
Templates with large-scale (1 mm2) perfect rectangular pore arrays without defect
TiO2 nanotubes grown in the template
(Before removing template)
Addressing system
Focus Ion Beam Lithography
Drawback:
1. Time consuming.
2. The pattern area is too small to the semiconductor process.
Visual operation
Addressing system
Electron Bean lithography
Alignment between the top of nanowire and the electrode lines.
Addressing system
Addressing system
We success realizing perfect match between the
top of nanowire and the electrode lines !
Addressing system
Then, how to realizing the backside electrode lines?
Al Al
PDMS
PDMS
Glass or Si
Glass or Si
Addressing system
Al Al
Ag 200 nm
Ag 10 um
Ag 200 nm
Ag 10 um
New process- a metal supporting layer
Addressing system
How to match the both side electrode lines as we design????
Front side Back side
Addressing system
We success realizing good matching between
the top and bottom electrode lines !!!
Thank you and have a nice day!