Solid and soft nanostructured materials: Fundamentals and applications

M. Willandera,*, O. Nura, Yu E. Lozovikb, S.M. Al-Hillia, Z. Chiragwandia, Q.-H. Hua,

Q.X. Zhaoa, P. Klasona

aPhysical Electronics and Photonic, Physics Department, Goteborg University, SE-412 96 Goteborg, SwedenbInstitute of Spectroscopy, Russian Academy of Sciences, 142190 Troitsk, Moscow District, Russia

Available online 3 June 2005

Abstract

The scientific work worldwide on nanostructured materials is extensive as well as the work on the applications of nanostructured materials.

We will review quasi two-, one- and zero-dimensional solid and soft materials and their applications. We will restrict ourselves to a few

examples from partly fundamental aspects and partly from application aspects. We will start with trapping of excitons in semiconductor

nanostructures. The subjects are: physical realizations, phase diagrams, traps, local density approximations, and mesoscopic condensates.

From these fundamental questions in solid nanomaterials we will move to trapping of molecules in water using nanostructured electrodes.

We will also discuss how to manipulate water (create vortices) by nanostructure materials.

The second part deals with nanorods (nano-wires). Particularly we will exemplify with ZnO nanorods. The reason for this is that ZnO has:

a very strong excitons binding energy (60 meV) and strong photon–excitons coupling energy, a strong tendency to create nanostructures, and

properties which make the material of interest for both optoelectronics and for medical applications. We start with the growth of crystalline

ZnO nanorods on different substrates, both crystalline (silicon, silicon carbide, sapphire, etc) and amorphous substrates (silicon dioxide,

plastic materials, etc) for temperatures from 50 8C up to 900 8C. The optical properties and crystalline properties of the nanorods will be

analyzed. Applications from optoelectronics (lasers, LEDs, lamps, and detectors) are analyzed and also medical applications like

photodynamic cancer therapy are taken up.

The third part deals with nano-particles in ZnO for sun screening. Skin cancer due to the exposure from the sun can be prevented by ZnO

particles in a paste put on the exposed skin.

q 2005 Elsevier Ltd. All rights reserved.

Keywords:: Nano-structures; Semiconductors; Soft materials; Trapping; Excitons

1. Introduction

At present the growth, processing and characterization of

nanorods and nano-particles is of global interest. There is an

intensive research for controlling and manipulating nano-

structured materials. This is of interest for both fundamental

understanding as well as for technical and medical

applications. This intense interest is true for both solid as

well as soft materials, or in some cases a combination of

both. The area of nanostructures and its applications is

rapidly expanding.

The quasi-two-dimensional system of spatially indirect

excitons in coupled quantum wells is perspective for the

0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mejo.2005.04.020

* Corresponding author.

E-mail address: mwi@fy.chalmers.se (M. Willander).

observation of coherent state and superfluidity [1–13]. The

last can manifest itself in the existence of persistent current

in each quantum well, quasi-Josephson phenomena in this

nonsuperconducting system, in statistics of fluorescence and

in variety of nonlinear optical phenomena (see [1–9] and

references therein). For extended 2D exciton system only

quasi-condensate with fluctuating phase and power-law

correlations is possible and the transition to superfluid state

is Kosterlitz–Thouless transition. However one can create

the trap for excitons analogeous to atom trap [14–16]. In this

case Bose-condensation can be observed in finite exciton

system even for two-dimensional case. In this connection

two main problems arose: the first is how to trap excitons.

There are several possibilities: the first is the trap (natural

quantum dot) originated from random fluctuations of the

potential connected with the roughness of interfaces in

quantum wells, impurities, defects, etc. (see interesting

experimental work [11] and analysis in [3] and references

herein). Other possibility is to create the mesa by nano-

Microelectronics Journal 36 (2005) 940–949

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M. Willander et al. / Microelectronics Journal 36 (2005) 940–949 941

technology. Finally, there is the possibility to create

confining potential by nonhomogeneous pressure near the

surface created by a needle. Using last method interesting

experimental results were obtained recently by Snoke group

[12].

Moreover, the research on soft materials has been

growing and in most cases it is interdisciplanry. Among

soft materials, water is being an important member. The

presence and wide usage of water in many chemcial

reactions and biological expriements, lead to the critical

need for a nano-scale water based platform for the

utilization of a wide range of sophisticated experiments,

e.g. trapping single molecule, studying chemical reactions

on few molecules level. Such experiments are not only of

fundamental interest, but have a strong technological

impact. We have developed a nano-scale water based

transistor [17]. This nano-device is in fact a unique flexible

platform with up to sixteen nodes connected to different

micro- and nano-active parts of a specific configuration.

Zinc oxide is a direct bandgap wurtzite type semicon-

ductor with energy gap of 3.37 eV at room temperature. It is

one of the important members of the wurtzite family. The

two important characteristics of the wurtzite structure are:

the noncentral symmetry and polar surfaces. Due to its large

bandgap, ZnO is an excellent semiconductor material for

applications considered for other wide bandgap materials

such as GaN and SiC. In addition to this, due to the extreme

large exciton binding energy (about 60 meV), the excitons

in ZnO are thermally stable at room temperature, and thus

ZnO has significant advantages in optoelectronic appli-

cations such as for ultraviolet (UV) lasing. Optically

pumped stimulated emission has been demonstrated

recently in ZnO nano-wires [18] and ZnO films [19–20].

Due the low symmetry of ZnO crystal, three free exciton

(FE) transitions can be observed in high quality ZnO

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

3400 3500 3600 3700 3800

R

Wavelength (Å)

FEA

FEB

Excited exciton statesof FEA and FEB

E⊥ c

T=80 K

Fig. 1. ZnO reflectivity versus photon wavelength in the exciton region for

E perpendicular to the c-axis at 80 K.

materials. Fig. 1 shows a typical reflectance spectrum of

bulk single crystal ZnO, measured at 80 K. The exciton

transitions are labeled as FEA, FEB and FEC. A number of

investigations on the synthesize of ZnO nano-wires have

been reported in the literature [21–24]. The traditional

problem in ZnO is to obtain p-type conductivity is still

challenging. Recent studies also indicate significant pro-

gress in the growth and understanding of p-type ZnO layers

[25–30]. These successes suggest the potential of ZnO for

applications in electronic and optoelectronic devices in the

near future. One-dimensional materials such as ZnO nano-

wires are of interest due to their importance in basic

scientific research and potential technological applications

[31]. These nano-wires can be grown at relatively low

temperatures and on wide choice of substrates, ranging from

Si to plastic (flexible substrates). ZnO nano-wires have

potential for applications in laser devices due to their

desirable optical properties. Therefore a detailed under-

standing of ZnO wires and the influence of impurities on

their properties are important. It may be worth noting that

ZnO nanorods as well as thin films can be grown or

deposited on various substrates including Si. We will show

our recent results for growth of nanorods in temperature

ranging from 900 8C down to 50 8C. In addition, ZnO nano-

particles are equally interesting and can be grown and

synthesized by different methods. These nano-particles are

of great interest for many technological as well as medical

appplications.

In addition, zinc oxide nano-particles have long been

recognized for its medicinal properties as an anti-irritant and

astringent as well as its UV blocking properties, in

sunscreens [32]. Scientists have shown that ultraviolet A

(UVA) radiation is a major culprit in photo-aging and skin

cancers [33]. Unfortunately, most sunscreens do not protect

against long-wave UVA. Ultraviolet radiation that reaches

the earth and damages skin can be divided into three key

wavelengths [33]: (1) Short-wave UVA (32–280 nm) or

UVC, (2) UVB (280–320 nm), and (3) Long-wave UVA

(320–400 nm) or UVA. We will present here our efforts to

characterize the influence of the particle shape (hexagonal),

and size (plate, equal ratio, column) on the optical properties

(scattering, absorption, and extinction efficiencies) for

moderate size diameter (DZ20–200 nm) in the UV region

(30–400 nm), comparable to the optically incident radiation

that dominant particle size range in sunscreens. We seek to

answer the following question: what is the effective

diameter of these hexagonal nano-particles that protect the

skin from the UVA, UVB, and UVC regions?

2. Excitons trapping

The width of a quantum well for excitons in the case to

create the confining potential by a needle the width of the

quantum well for excitons in this case is much larger than all

other scales in the problem [12]. In this case we can use

Fig. 3. Confinement potential for interwell excitons originated from non

homogeneous field created by STM for various forms of the tip: (1) conic

with the radii of the curving 20, 50, 100 and 200 nm, (2) parabolic with the

radii of the curvature (in lower point) 20 and 50 nm, (3) spherical electrode

with the radii of the curving 20 and 80 nm, (4) hyperbolic form of the tip.

M. Willander et al. / Microelectronics Journal 36 (2005) 940–949942

quasi-local approach. For rarified exciton system general-

ization of Gross–Pitaevskii equation can be used which

takes into account the summing of all ladder diagrams

(because the vertex for rarified Bose gas can be reduced to

amplitude of two-particle scattering but the last in 2D case

diverges at small energies—see, e.g. analysis in [2]):

KV2

2mCVextðrÞKm K

4pjfðrÞj2

m ln ðjfðrÞj2a2Þ

� �fðrÞ Z 0

The profile of the density for quadratic confinement

potential is slightly differs form Gaussian. For more

complex case, higher exciton concentration, we propose

another approach—quasi-local generalization of Koster-

litz–Thouless theory for the system. We also accept that

the width of quantum well for excitons is much larger

than all other scales in the problem (particularly,

separation between thermodynamically equilibrium vor-

texies in exciton system, etc.). The excitons have some

profile n (r) with maximum in the center of the (axially

symmetrical) trap and zero on the boundaries. Let

temperature T to increased. For any given temperature

T it can be equal to ‘local’ Kosterlitz–Thouless critical

temperature [34] which is proportional to the density

in the same point R. Due to axial symmetry all the

circle with radius R will be superfluid. At the boundary

R universal jump of superfluid density to zero takes

place.

So the ring at rOR will be normal (see Fig. 2). When

temperature rises the superfluid circle shrinks to zero at

critical temperature Tc which is proportional to the density

nmaxtot in the center of the trap.

The radius of the superfluid exciton system in the trap is

rs Z Lffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 KT =Tc

p

where L is Thomas–Fermi radius.

Sufficiently narrow exciton traps can be created be higly

nonhomogeneous fields U(r) of the STM tip. For spatially

indirect excitons the confining potential is

Fig. 2. Superfluid circle and normal ring for exciton system.

Ueff Z dvU

vz

where d is dipole moment of indirect exciton due to charge

separation. The resulting confining potential calculated for

different forms of the tip is presented on Fig. 3.

The results on Fig. 3 were calculated for the system

presented on Fig. 4. If the doped layer is present on the

surface it can screen the electric field of the STM tip and

confining potential for excitons disappears. But if the

current is presented in tip–semiconductor system (due to

electron tunneling) the confining potential appears as the

current grows.

The exciton system confined by STM tip has small sizes,

of order of 1 m. So if the exciton density has the order of

Fig. 4. Radial distributions of condensed and noncondensed particles at

different number of particles N. Mesoscopic supersolid regime.

M. Willander et al. / Microelectronics Journal 36 (2005) 940–949 943

1010 cmK2 or smaller the number of excitons is smaller than

100, i.e. exciton system is mesoscopic. We studied the

Bose-condensation and strongly correlated regime in

mesoscopic exciton system. When the dipole moments of

the system grow (or confining potential diminishes) the

profile of the system changes essentially: it has Gaussian

form for weakly interacting system, then when interaction

grows it has the form of inverted parabola. For even more

strong interactions the profile of the total density and profile

of Bose–condensed particles have shell structure. By other

words in the mesoscopic Bose system beside superfluidity

some crystal-like order appears and the system become

mesoscopic supersolid (Details will be published elsewhere;

see also [35]).

Fig. 5. The water transistor, (a) A microscope photograph showing the large

area of the device with a water drop placed on top covering all active device

area, (b) the large pH electrodes (200!200 m2), and (c) the small pH

electrodes 0.6!0.6 mm2 together with the nano-gap (different configur-

ations with gaps of 20–200 nm) sensing electrodes [17].

3. The nano-scale water transistor and applications

We have developed a nano-scale water based Si

compatible water transistor [17]. The goal of the device is

to trap a single molecule in water and at the same time tune

the water pH. The physics behind the trapping is the same as

for excitons, i.e. using the gradient field. The active area of

the device composed of two (or more) nano-gap electrodes

(200 nm and down to 20 nm gaps have been processed) and

a third nearby electrode for controlling and manipulate the

pH of the water. The nano-gap electrodes play the role of the

emitter and collector, while the third electrode is considered

as the base in analogy to the bipolar transistor convention.

The operation of the device is described by a bipolar

transport. It is very sensitive to pH variations. In fact there is

a vast need for miniaturized pH sensors in many

biochemical, chemical or industrial applications, since

most of chemical or biological processes are pH dependant.

The ability of the developed device to detect local variation

of the pH, imply that, it is in fact a unique platform for a

many interesting experiments for both fundamental under-

standing as well as for the possible great technological

impact. Below we will briefly describe the principle of

operation and show some of the DC characteristics.

The nano-device was fabricated using silicon compatible

technology combined with electron beam lithography. A

low n-type doped Si wafer was used as the substrate for the

device. An 80 nm thick thermal oxide was grown on the

wafer to provide isolation. Different electron beam

lithography steps were performed to produce different

configurations. The final step was an insulting layer with

open windows on both the tip of the nano-electrodes as well

as for the pH sensing electrodes. Both the gap between the

nano-electrodes as well as the position of the third pH

electrode with respect to the sensing nano-electrodes and its

size were designed in different flexible combinations. Each

device contains 16 nodes. To investigate the pH response we

put a drop of de-ionized clean room water (resistively of

18 MU) on the sensing area of the device. The base

electrode, as well as the emitter and collector were biased in

the common emitter configuration. Fig. 5 shows different

micrographs and scanning electron micrograph of the

device. In Fig. 5(a) a microscope micrograph showing the

large pH electrodes (200 ! 200 mm2) with the water drop

covering all the active area of the device. Fig. 5(b) is an

SEM showing the pH electrodes of Fig. 5(a). While Fig. 1(c)

shows the small pH electrodes (0.6 ! 0.6 mm2) together

with four nano-gap sensing electrodes. Note that the small

electrode part of the device is located inside a small part of

the middle dark area (80 ! 80 mm2) of Fig. 1(b) as indicated

by the arrow. The three terminal device (transistor) basic

operation is based on the variation of the pH of water by

applying a voltage and independantly measure the current

variation between the two nano-spaced electrodes. From our

Fig. 6. The water transistor output characteristics (IECKVEC for different

VBE biases). This output characteristics were measured using the small pH

electrode configuration shown as insert [17].

M. Willander et al. / Microelectronics Journal 36 (2005) 940–949944

analysis of different configurations, we found that the

magnitude of the transistor current and sensitivity to pH

variation depends a lot on the configuration used with regard

to the positioning and distance of the base (pH control

electrode) from the sensing area (emitter–collector nano-

electrodes). When a voltage is applied between the emitter

and base (VBE), the water decomposes and the pH of the

water changes. In presence of an electric filed between the

emitter and collector (VEC), the OHK ions will be attracted

to the collector, and a potential drop is established between

the OHK and their images at the metal. Here the charge

neutrality is postulated for the whole system and, therefore,

OHK ions have to be compensated for by oppositely

charged ions, namely the HC ions in the water. The

dehydrated ions (OHK) are located in a plane adjacent to the

collector electrode called the inner Helmholtz plane (IHP).

The hydrated ions (H3OC) which diffuse from the bulk will

be located in the so called outer Helmholtz plane (OHP)

[36]. The H3OC ions are then surrounded by water dipoles

and will become nonconducting species. They cannot enter

the outer layer, and hence a potential drop is then

established between the OHP and the metal. The total

charge per unit area on the metal (s) is given by the

electronic charge multiplied by the difference between the

number of anions (OHK) and cations (HC) per unit area

[36]. Using the geometrical capacitance between the metal

and the OHP, and proceeding to establish a simple

expression to estimate the current between the emitter and

collector when a voltage of VEC is applied between them, an

expression for the emitter to collector current can be

obtained [17]. Quantitatively, this expression contains two

different dependencies that give the magnitude of the

current IEC. The first is a geometrical dependence. The

second is the effect of the third electrode. In deriving this

expression, we only considered decomposition of water, and

we have ignored other reactions that might exist [37–38].

These dependencies are in fact in consistency with our

experimental observations. In general, the main features of

the I–V output characteristics are: (1) asymmetric I–V with

regard when comparing the forward and reverse biasing (2)

variation of the emitter-collector current by changing the

emitter-base voltage, and (3) shift of the threshold voltage.

The shift of the threshold voltage is expected due to the pH

change. Fig. 6 depicts the output characteristics of the nano-

scale water transistor using the small pH electrode

configurations shown as insert in the figure. This pH

electrode configuration is special in the sense that the pH

electrodes electric field is lying in the same direction as the

field produced between the emitter and collector as can be

clearly seen. As clearly seen, the I–V is asymmetric in

behavior as expected. In addition, a shift in the threshold

voltage is observed in the reverse direction. In addition, a

plateau (saturation) is observed. The origin of this is

currently under investigation. While in the forward the

emitter–collector current response to the pH variation is a

weak response. This observed I–V behavior is mainly

attributed to the special location of the small H electrodes

with regard to the emitter–collector electric field. This

indicates the important role of the design of the pH electrode

with regard to its location and size. Although all other

analyzed configurations showed asymmetric I–V behavior,

all showed sensitivity to VEB on both polarities and a clear

shift of the threshold voltage was observed.

The developed nano-scale water transistor is a suitable

platform for many applications. The fact that local pH

variations can be monitored implies that the device can be

used to sense any type of hydrolytic enzyme reaction. The

presence and biocompatibility of water will open the door

for many applications. As an example we will emphasize

and briefly discuss trapping single molecule reactions.

Conversion of chemical energy into electrical signal in

hydrolytic media combined with addition of fluorescent

markers can lead to a platform of studying and analyzing

‘few’ trapped molecules. In fact trapping of small molecules

in aqueous media has become very interesting due to the

wealth of information possible to obtain from such

experiments. Spatial manipulation of objects by AC

electro-kinetics is a well established technique [39]. In an

inhomogeneous electric filed, polarizable particles experi-

ence a lateral force towards the regions highest filed

strength. This force is called dielectrophoretic force and it

depends on the dielectric properties of both the particle and

their surroundings. Employing alternating field, electro-

phoresis effects are avoided and permanent charges remain

unaffected. Objects that are smaller than about a micrometer

have come into focus only in recent years. This is because

the dielectrophoretic effect is proportional to the cube of the

particle’s radius, while the impact of the Brownian motion

increases with decreasing particles size. Therefore very high

electric field gradients are needed, which, on the other hand,

lead to disturbance by heating and electro hydrodynamic

M. Willander et al. / Microelectronics Journal 36 (2005) 940–949 945

effects. This dilemma is solved by working in low

conductive media and by using electrodes with gap widths

of only a few micrometers or less. Recently electric

properties of viruses and polystyrene micro spheres have

been determined by studying their edges or dielectrophore-

tic response at different field frequencies. Macromolecules

of some micrometers length like DNA has been concen-

trated at the electrode have been aligned in parallel to the

electric filed. In order to manipulate smaller molecules,

especially more compact ones like proteins, even higher

electric field strengths or higher field gradients, respect-

ively, are necessary. Interdigitated electrodes with thickness

of typically 100 nm exhibit a gradient almost only in the z-

direction. Thus sharp electrodes with a shape similar to

needles help to increase dielectrophoretic. Field strength

can be increased by using higher voltage or smaller

electrode distances. Side effects like heating can be reduced

by decreasing the volume of high fields. This is achieved by

shrinking the active volume by reducing electrode distances

and by confining the region of highest field strength to a

small region by, again, sharp electrode tips. The developed

platform, i.e. the nano-scale water based transistor, is an

ideal choice for trapping small molecules other biological or

chemical species. Indeed the presented platform was used

for such purpose. We have employed many nano-eletrodes

with different nano-gaps and different sharpness. Exper-

iments were performed on fluorescently labelled IgG

antibody molecules with a molecular weight of 150 kDa,

corresponding to a diameter of about 8 nm. The “north” and

“south” electrodes were activated by 3 V (RMA) at 100 kHz

(see [40]).

Moreover, we have also used the nano-scale water

platform to study and control vortex flow in a chemical cell.

We have analyzed and described the vortex phenomena. We

showed that in pure water and under external static electric

field we can reproduce and control the vortex flow

formation in a chemical cell (water transistor platform).

The origin of the phenomena is due to the electrochemical

decomposition of water. Due to the low conductivity of pure

water in absence of electrolyte (18 MU), the field driven

hydroxide ions at the anode becomes essential to the proton

release, which in turn is the result of the molecular O2(gas)

evolution. Water recombination processes, which have

proton flowing in hydroxide, background as a key ingredient

produce the phenomena of vortex flow [41].

4. ZnO nano-structures synthesis

As mentioned in the introduction, ZnO nano-structured

materials have attracted intense research communities

recently. The reason is that ZnO is a unique material.

Beside the physical properties mentioned in the abstract,

ZnO exhibits dual semi conducting as well as piezoelectric

properties. Nano-structures possibly to obtain from simply

ZnO powder, is by far more diverse than any known nano-

material including carbon nano-tubes. By solid state thermal

sublimation process with control of the growth kinetics,

local growth temperature, and the chemical composition of

the source material, many nano-materials can be obtained

with 100% reproducibility. Some of these are nanorods,

nano-belts, nano-rings, and more complicated nanostruc-

tures like nano-propellers with different equivalent multi

crystallographic directions, etc. It is important to mention

that all the possible ZnO nano-materials are single crystal-

line and defect free. We will restrict the material in this

paper to ZnO nanorods synthesized by a vapour phase

transport process using Au nano-particles as catalyst [18]. In

this growth procedure, thin films of Au were deposited on a

substrate (1–2 nm), which will then placed in a close

vicinity of a mixture of ZnO and graphite powder in a tube

furnace. The furnace is heated to about 900 8C for 30 min

and is then cooled to room temperature in the flow of argon.

In this process, ZnO is reduced by carbon; simultaneously

Au film de-wetted the substrate and formed nano-clusters.

When Zn atoms condensed on Au clusters, Zn formed alloy

with Au temporally and rendered AuZn in a liquid form.

Under the catalysis of Au, Zn is oxidized to form ZnO with

the Au clusters being elevated on top of the ZnO. Since the

lateral growth of ZnO was limited by the size of the Au

cluster, thin rods would result as time passed by. The

pronounced growth direction is !0001O. The orientation

of the rods with respect to the surface of the substrate was

determined by the orientation relationship of the ZnO!0001O and the crystallographic orientation of the substrate.

In addition, we have also developed a relatively low

temperature (around 50 8C) process based on chemical

reactions to synthesize high quality ZnO nanorods. The

main application for such low temperature process could be

nano-photonics devices on plastic (flexible substrates, e.g.

for electronic cards). In Section 5, some of our recent results

obtained from samples grown using this low temperature

techniques will be briefly described.

Fig. 7 displays different scanning electron micrograph

with different magnifications of a variety of ZnO nanorods

grown on Si, SiC, both at temperature around 900 8C and

finally on ITO at 50 8C. The hexagonal nature of the

nanorods is clearly seen. A variety of substrates have been

successfully employed to synthesize high quality ZnO

nanorods. Among them, oxidized Si (001), Si (001), Si

(111), sapphire (0001) and (1 1 K2 0) and amorphous SiN

membrane were used as substrates for the ZnO nanorods

growth. As mentioned above ITO (p-type) was also

successfully used for the growth of ZnO nanorods. The

choice of the Si substrates is made with the intention of

integrating the ZnO nanorods with silicon technology. The

choice of the sapphire substrate is to obtain vertical growth

of the nanorods. The choice of SiN membrane is made in

order to utilize TEM (transmission electron microscopy not

shown here) to study the structural properties of individual

rods on the substrate. We have been able to vary to diameter

of the ZnO nanorods in a typical growth procedure down to

Fig. 7. Different SEM of (a) nanorods grown on Si (001)-substrate at 920 8C. The length of ZnO nanorods is about 2 mm with diameter of about 0.3–0.8 mm, (b)

SEM viewing single hexagonal ZnO nanorods grown on 4H-SiC substrate at 900 8C, and (c) SEM image of ZnO nanorods grown on ITO-deposited glass

substrate at 50 8C in a solution.

5.00

10.00

15.00

3600 3700 3800 3900 4000 4100

PL in

nten

sity

(ar

b. u

nits

)

Wavelength (Å)

ZnO wireson

EVI

EVI

-1LO

FEA

D0 X T=80 K

Si

Sapphire

Fig. 8. PL spectra from ZnO wires grown on sapphire and Si substrates,

measured at 80 K.

M. Willander et al. / Microelectronics Journal 36 (2005) 940–949946

100 nm. Photoluminescence (PL) spectra were acquired for

all samples. We have reproduced most of the features of

the spectra reported in [18] except for the lack of the

appearance of the sharp peaks due to the lower power

density of our laser. In addition, some of our samples shows

strong white-bluish illumination witnessed by the naked eye

and signified by a broad peak starting at 4000 A and ending

at 6500 A with maxima located at about 4800 A. The PL

measurements were carried out in the temperature range of

80–300 K. A double grating monochromator and a photo-

multiplier detector were used to disperse and detect the ZnO

emission. The laser lines with a wavelength of 270 nm or

350 nm from an ArC laser were used as the excitation

sources. Fig. 8 shows the PL spectra of ZnO wires grown on

sapphire and Si substrates at a temperature of 80 K. The

dominant transitions are the free exciton (FEA) related to an

impurity bound exciton (BE) that is likely due to donor

bound excitons and a further intense transition labelled as

EVI. By carefully examining the wavelength range longer

than the EVI transition, we observed the LO-phonon replicas

of the EVI transition with up to three LO-phonons involved.

In the PL spectrum shown in Fig. 8, only the first LO-

phonon replica is clearly visible due to the large intensity

scale. The energy separation between the FEA and EVI is

about 60 meV at the temperature of 80 K, and this energy

separation decreases with increasing temperature. ZnO

wires grown on sapphire and Si substrates show a similar

spectrum, except that the FEA is relatively stronger in the

sample grown on Si substrates, which indicates that the

background doping concentration is relatively low. From

0

200

400

600

800

25 30 35 40 452 theta (deg.)

Inte

nsity

(cou

nts)

ITO

(22

2)

ZnO

(002

)

ZnO

(100

)

ZnO

(101

)IT

O (

400)

Fig. 9. Powder X-ray diffraction of ZnO nanorods in Fig. 1A (c).

M. Willander et al. / Microelectronics Journal 36 (2005) 940–949 947

the PL spectra measured at different excitation power, we

can state that at the excitation power used here, none of

the observed transitions are related to inelastic exciton–

exciton scattering. As ZnO is n-type in an un-intentional

growth process, the growth on the p-type ITO was for the

purpose of obtaining a heterostructure pn junction for

electrical characterization. Fig. 9 is a typical X-ray spectrum

obtained from the growth on ITO at low temperature.

Although this X-ray diffraction spectrum indicates a non-

axis growth, it shows the appearance of very clear and

strong peaks of single crystal ZnO rods. This implies that,

our newly developed process is very promising and more

optimization can lead to the growth of on-axis single crystal

ZnO nanorods of high quality. Fig. 10 displays the

Cathodoluminescence (CL) of the same sample. The spectra

consist of two features, a sharp exciton peak at 380 nm

reflecting the recombination of electron beam induced

electron-hole pairs in the crystalline ZnO nanorods, and a

Fig. 10. Cathodoluminescence of the ZnO nanorods shown in Fig. 8 (c).

broad visible peak at about 550 nm. The large difference in

the intensity of the two peaks seems to indicate the high

quality of the sample.

Beside the obvious application of ZnO nanorods for

nano-photonics, we will briefly describe here the possibility

of employing ZnO nano-materials for medical cancer

photodynamic therapy (PDT). The technique of the PDT

dates back to 1903, where the first trial was reported [42].

However, since then, the technique has been refined a lot.

Nevertheless, the main steps involved are still the same. The

PDT method relies on the coexistence of a photosensitive

compound (photosensitizer), oxygen and light. The photo-

sensitizer is administered to the patient, where it accumu-

lates in the cancerous tissue. The main features of the

technique are: it’s high selectivity, relatively fast healing

rates, and the ability to treat the same tissue several times if

needed. The mechanism of the PDT is based on the fact that

the therapeutic light, which must match absorption band of

the photosensitizer, excites the photosensitizer molecule

from its single ground state to its first excited singlet state.

The molecule can then, with high probability, be transferred

to its first excited triplet state. This transition although spin

forbidden, but its probability is rather large due to the small

separation between these two states. The final relaxation to

the ground state will also be spin forbidden, leading to long

life time (R100 ms). This long relaxation life time imply a

high probability for interaction with the surrounding

molecule. The excess energy of the photosensitizer may

be transferred to oxygen molecules, which thereby are

excited from their triplet ground state to one of the first

excited states that are biologically active. Singlet oxygen is

highly cytotoxic and its formation leads to degradation of

the cancerous cells by various mechanisms. The PDT is

selectivity leads to local treatment with low risk of damage

of healthy cells. Moreover, it is very efficient in also

treatment of precancerous cells. ZnO with its discussed

photonic properties is a very advantageous for the purpose

of PDT. In this connection, we will below also present our

recent work to optimize the use of ZnO nano-particles for

sun screening from the damaging part of the sun ultra-violet

radiation.

5. ZnO nano-particles UV absorption and scatteringefficiency

To describe light scattering from an arbitrarily shaped

nano-particle, we have adopted (DDA) method, as first

formulated by Purcell and Pennypacker [43] and modified

by Draine [44] and Goodman et al. [45]. In this method, the

particle of interest is consisting of array of N electro-

magnetically interacting dipoles on a cubic lattice grid.

Each dipole radiates a dipole field in response to the incident

radiation and the radiated fields of all other dipoles in the

ensemble. We have used DDSCAT6.1 program written by

Draine and Flatau [46] and we have modified their code to

Fig. 11. (a) Hexagonal sphere-packing of the ZnO particle model, (b) plate-

like shape, (c) column shape, and (d) real plate ZnO particle synthesized by

the vapor phase transport process as described in the text.

Fig. 12. Absorption and scattering efficiency factors versus wavelength for

plate, equal ratio, and column hexagonal ZnO particle with two constant

effective radii (40 and 80 nm).

M. Willander et al. / Microelectronics Journal 36 (2005) 940–949948

generate hexagonal particle geometry of identical spheres

from the planer generation matrix [47], then to generate

three-dimensional hexagonal shapes in a fundamental

region by extending the planar hexagonal lattice as a

building block for the first layer to any vertex–vertex

diameter of the hexagonal face and repeated this layer many

times fills the whole space of the final three-dimensional

hexagonal particle. This situation is pictured in Fig. 11.

Rather than direct methods for solving set of 3N complex

linear equations, in this study we fellow Flatau [48] in using

the stabilized version of Bi-Conjugate Gradients (Bi-

CGSTAB) method [49] with preconditioning. We adopted

the Draine and Goodman [45] ‘Lattice Dispersion Relation’

(LDR) method for prescribing the dipole polarizabilities.

The constraint for the validity of the DDA code is distance

between neighbouring dipoles; dZ(4p/3)1/3R, where R is

the radius of the dipole; must be small enough compared

with the wavelength l of light in the surrounding target

medium and for accurate calculations we need a more

conservative criterion kdjmj%0.5. In our study we specify

bZ0, fZ0, and QZ08, 608, 908 and we choose unpolarized

incident light for our study. Once the polarizations are

known, the extinction Qext, absorption Qabs, and scattering

Qsca efficiency factors may be evaluated from the optical

theorem. We take relative refractive index; which is the

ratio between the refractive index of the particles (ZnO) and

that to the matrix, such as the oil phase (in our study we take

it water for simplicity), we simulate the data for real and

imaginary parts of refractive index by adopting the work by

Dakhel [50] as approximate data and we used Segelstein

[51] data for the real refractive index of water in the UV

region; and wavelength in the medium which is the ratio

between the wavelength in vacuo to the real refractive index

of the medium.

Three different particle shapes are considered in this

study: hexagonal plates with aspect ratio (0.6650), hexagonal

particles with aspect ratio (1.0006), and hexagonal columns

with aspect ratio (1.4963) for values of the size range reffZ10 nm(10 nm)100 nm. The comparisons between absorption

and scattering efficiency factors for ZnO particles for three

hexagonal shapes with same effective radius are shown in

Fig. 12, the results for effective radiusZ40 and 80 nm. For

each size, both figures display two bands of similar position,

intensity and shape for the plate and equal lengths hexagonal

particles with small difference between the values for the

column shape. The similarity between the plate and equal

lengths hexagonal particles came from aspect ratio assumed

for the plate (0.6650), this value get less effective with

decreasing the effective radius as we can see from the curves

of effective radius 40 nm, we found it much similar and have

the same values in most regions than the curves of 80 nm. The

small difference in intensity between the plate and column

hexagonal particles arises obviously from the difference of

the target frame axes of the plate and column hexagonal ZnO

particle (see Fig. 1) and of course from the different in shape

of plate and column. When the particle size increases, its

shape being kept constant, the only increase the peak

intensity, which in this case is proportional to the hexagonal

volume. The location and the intensity of the first and second

peaks in the Fig. 12, which shifted are dependent on the

wavelength and particle size. To explain this case, the first

peak in the UVC region arises from the dipole excitations and

the relation between the polarizability and dielectric function

to extract 3(u) [52] and the second peak in the UVA

associated with the band gap at (380 nm) for the bulk ZnO

and this peak is shifted to the shorter wavelengths as the

M. Willander et al. / Microelectronics Journal 36 (2005) 940–949 949

particle size decreased because the wavelength dependence

of the absorption cross section seems to depend on the mean

optical thickness of the hexagonal nano-particle which is

function of the N and l [53].

6. Summary

In summary, we have presented some different important

issues related to nano-structured materials. Both funda-

mental as well as technological aspects are presented and

discussed. Some recent results of exciton trapping are

presented (e.g. the superfluid mesoscopic system). The so

called nano-scale water transistor platform developed

recently is presented. Applications of this unique pH local

sensor are discussed. We emphasized the use of the platform

to trap and study single or few molecules. Advantages,

growth, and characterization of ZnO nano-wires together

with our recent results are discussed. We have developed

relatively low temperature process (50 8C). Such a low

temperature process is of great technological impact due to

the possibility of growth on flexible substrates (e.g. plastic),

which is of interest for plastic electronics card. Photo-

dynamic therapy is suggested to benefit from ZnO nano-

materials due to the suitability. In addition, we have

developed a model, based upon light scattering by single

particle, which enables us to account satisfactorily for the

absorption, scattering, and extinction spectra of hexagonal

ZnO particles. We have studied the effect of the particle size

on scattering and absorption, with the aim to optimize the

processes for sun-screening against skin cancer.

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