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One-Dimensional ZnO Nanostructures: Solution Growth and Functional…
One-Dimensional ZnO Nanostructures: Solution Growth and
Functional Properties
Methods
methods
PVD and flux
high temperature, impurities problem
cannot integrate with organic folded substrates
Metal-organic CVD and MBE
poor yeild and uniformity, limited substrate choice
Pulse laser deposition, sputteting, top-down
less control and reproduction
Wet chemical
low cost, less dangerous, low temp - compatible with organic sub, no metal catalyst, integredable with silicon tech
lots of tunning parametres - variety of morphology
Properties
colorless due to 3d10 config
zn2+ Exist in several intermedies depending on pH, and forms zno by dehydration
hydrolysis of Zn salts in basic solution
Growth mechanisms
ZnO as wurzite growth along c axis because of high free energy of polar surfaces
polar surfaces adsorb ions of different polarity over and over thus [0001] (polar) growth faster than [1-100] (non polar)
Growth in alkaline solutions
divalent atoms do not hydrolyze in acidic solutions
use of KOH of NaOH
KOH is better, it has larger ion radius and lower probability to incorp in ZnO lattice
reactions
in reactions (
p 3
) O2- comes from base, not from H2O, so solvent can be ethanol, methanol, ionic liquids
different intermediates form with different concentrations depending on pH
under alkaline conditions growth can take place under room temp
morphology is controled by tuning oh- concentrations and reaction time
Aspect ration
increases in less polar solvents
polar molecules of solvents interact with polar planes, so they grow less fast and srtucture becomes more "fat"
in nonpolar hexane nanowires was 2nm in diameter
possibly formes by oriented self-assembling of preformed ZnO QD
the holes between QD were later filled by Ostwald ripening
GO TO SEEDING
use of NH3*H2O
weak base, controls saturation of solution, allows to supply only heterogeneous growth (on substrate) and avoid actual nucleation in solution
that is why after growth and during it solution has to be clear
O2 role
if h2o2 was added in solution
it decomposed on h2o and o2
zno with sharp surface is formed
if extra dissolved o2 was elimenated
zno with raged surface is formed
Growth with HMTA
role of HMTA
pH buffer
weak Lewis base
coordinates Zn2+ ions
attaches to nonpolar facets and enhances anisotropic growth along[0001]
it hydrolyses and produces HCHO and NH3
as it is a base - sourse of oh- ions
If HMTA just dissosiates ZN2+ ions will quickly precipitate, nutrient in solution will run out
The oriented anisotropic growth will be disrupted
role of NH3 (as HMTA decomposition product)
provides basic environment (necessary for zn(oh2) formation
coordinates with zn2+ ions
reactions are reversible and in equilibrium
precursor concentrations
nanowite density
can be controlled by tuning
growth time, temperature
determine morphology and aspect ratio
role of side products
acetare, chloride, formate form rods
nitrate, perchloride form wires
sulfate form flat hexagonal plates
GO TO SEEDING
seeded growth
allows to use various substrates (flat, etched, flexible etc.)
to improve adhesion use
intermediate metall level (Cr or Ti) for inorganic substrates
interfacial bonding layer (tetraetoxysilane) for organic substrates
how to obrain
sputtering of bulk matetial
spin coating of colloidal QD
use metalic Zn
it easily oxidazes in air and solution
mechanism
formation of nanowire usually occures between two QD grains
that is why the average diameter of so-growth NW 100 nm
the length can be 1 mkm
aspect ratio of about 100
due to pollycrystalline nature of seed the vertical orientation is always poor, also there is a paper telling it is achievable
the alignment strongly depends on humidity level
activation energy is lower in heterogeneous growth
so seeded ZnO NW will grow at lower level of supersaturation of solution
so NW will growh on the substrate whether there is seeds or not
because nucleation still will be less favorable
but on Si it results in poor alignment because of large lattice mismatch
density of ZnO NW (per area)
the thickness of films controls NW density
tuning QD film thickness can be performed by changing spin speed (
p 7
)
if seeds do not form continious layer, the thickness of seed film is small
add layer of polymer above the seed layer
it will act as an obstacle for source molecules diffusion (
p 7, art 115
)
epitaxial growh
similar to seeded growth
use materials with small lattice mismatch (Au, Cu - 12 %)
Caping agent
two types
adsorbs onto side planes and enhance vertical growth (PEI, ethylenediamine
PEI consists of large number of NH2 groups, which protonate and become positively charged
protonated positive PEI molecules adsorb onto lateral surfaced and inhibit lateral growth
also helps to lengthen NW by extending growth time
similar to adding NH3*H2O, PEI coordinates Zn2+ ions and decreases the precipitation of Zn(OH)2 - Zn2+ more likely to become part of growing NW
adsorbs onto basal plane and enhance lateral growth (Cl-, citrate ions)
as a result - flat hexagonal plates
more polar planes exposed and more rough surface
enhanced photocatalytic activity compared to 1d NW
has 3 negative charge
different structures
belts
growth along nonpolar facets
not favorable in chem wet metods due to high free energy of polar surfaces
rectangular NB were formed by reverse microemulssion method
ZnO nanoparticles oriented attachment occured by adjasent nanoparticles fusion
then Ostwald rippening smothened side surfaces
rectangular NB were formed by simple alkaline method adding ethylenediamine
Tubes and rings
possible applications
high-efficiency solar cells
large internal area
gas sensors
well defined adsorbtion microcavities
growth
one step
two step - growth + etching
methods of fabrication (
p 14
)
tuning sees layer thickness
using solvent composition
ultrasonic pretreatment of growth solution
post pH adjustment
Formation mechanisms
via etching
polar basal surfaces undergo selective defect etching by protons
polar surfaces have higher energy, so etching speed in this direction is faster
distribution of defects: more in centre of [0001] and less in periphery, so center etches faster
hypothesis was indirectly confirmed: annealed ZnO wasn't etching
different termination atoms affect NT formation
when solution reaches equilibrium the formation of ZN(OH)2 is no longer beneficial and zno as a NW starts to dissolve
rings via etching and micelles
twinning structures
Hierarhial structures
used citrate ion as a caping agent, it adsorbes onto balal planes and inhibits growth along [0001]
secondary nucleation started by
another layer of seed QD
diaminopropane
it coordinates with water and releases OH- ion, increasing local pH
this leads to second layer formation
density of brances depends on diaminopropane concentration
by starting secondary nucleation
or hollow spheres
self-assembling of nanoclusters, stablized by PSS, nanorods
