You Qiang

Department of Physics, University of Idaho, Moscow, ID 83844-0903

IMR-Shengyang, 07/13/2007

Doped Semiconductor Nanoparticles:Doped Semiconductor Nanoparticles:Ferromagnetism and Photoluminescence above Room TemperatureFerromagnetism and Photoluminescence above Room Temperature

1

• Interest in wide bandgap semiconductor nanoclusters: TM-doped ZnO

• Experimental setup and synthesis of nanoclusters• Magnetism: analysis and discussion• Photoluminescence: analysis and discussion• Summary

Outline

References1. J. Antony, Y. Qiang, et. Al., “Room temperature ferromagnetic and UV optical properties of Co-doped

ZnO nanocluster films”, J. of Appl. Phys., 97, 10D307, 2005.2. J. Antony, Y. Qiang, et. Al., “ZnO Nanoclusters: Synthesis and Photoluminescence”, Appl. Phys. Lett, 87,

241917, 2005.3. J. Antony, M. H. Engelhard, D. E. McCready and Y. Qiang, “Anomalous double-exchange enhanced

ferromagnetism in transition metal-doped ZnO nanoclusters above room temperature”, Phys. Rev. Lett, accepted, 2007.

4. Jiji Antony, and You Qiang, “Ferromagnetism in Ti-Doped ZnO Nanoclusters above Room Temperature”, IEEE Transactions on Magnetics, 42 (2006) 2697.

5. Jiji Antony, You Qiang, et. al., “Ferromagnetic semiconductor nanoclusters: Co-doped Cu2O”, Appl. Phys. Letters, 90, 013106 (2007).

Spintronics: spin transport electronics

Information storage & processing on same spintronic chip

Interest in doped ZnO nanoclusters

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ZnO is a wide bandgap semiconductor that has attracted tremendous interest as blue light emitting materials, gas sensors and transparent conductors in solar cells.

1. “Green Technology”. Unlike other semiconductor compounds that contain cadmium, arsenic, or other environmental toxins, zinc and oxygen are “environment friendly” elements. In fact, ZnO is used as a dietary supplement in animal feed.

2. Energy efficiency. The stability of excitons in ZnO results in a very high quantum efficiency at temperatures of 300 K and higher. It is an ideal material for room-temperature magneto-optic applications.

3. Transparency. At RT the bandgap of ZnO is 3.4 eV, which is the UV region of the spectrum, making it an optically transparent semiconductor. Transparent ferromagnetic memory devices could be integrated with transparent transistors, providing “invisible” computing systems.

ZnO: desirable properties for device applications

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1. electronic and optical properties can be tuned by varying nanocluster size.

2. nanoclusters are embedded in a matrix to make nanocomposites or films.

3. by reducing dimensions to the nanoscale, high storage densities may be achieved by addressing the spin states of single quantum dots.

From a fundamental point of view, the study of nanoclusters provides insight into the mesoscopic regime between single molecules and bulk crystals. Nanoclusters exhibit strong quantum confinement and surface effects. Using nanoclusters as “building blocks” for novel materials offers the opportunity to understand how atomic structure can lead to physical and chemical properties of macroscopic materials. Moreover, it is possible to produce nanoclusters with atomic arrangements that do not normally exist in nature, leading to novel optical and magnetic properties.

Advantages of nano-semiconductors over bulk materials

6Theoretical work has predicted ferromagnetism above RT for Mn-doped ZnO (given a large hole concentration) . Spurred by that prediction, research into ZnO crystals for spintronic applications is an active area of experimental research.

1. No systematic study of size and dopant-concentration dependence on magnetic and optical properties of nanoclusters has been undertaken experimentally.

2. The fundamental magnetic and optical properties of TM-doped ZnO nanoclusters remain essentially unexplored.

3. Most recent reports have focused on the synthesis, structural properties, UV-visible optical, and magnetic properties of ZnO-based on thin films and bulk materials.

4. Dr. Gamelin’s group at the University of Washington has studied ZnO and TiO2 DMS nanoparticles synthesized using a colloidal method.

There is a significant lack of knowledge about the fundamental properties of doped semiconductor nanoclusters.

Interest in TM-doped ZnO Nanoclusters• Ferromagnetic semiconductor: applications in spintronics and

nonvolatile memory storage.• Manipulation of spins: memory storage efficiency increases.• Ferromagnetic behavior in DMS: optical devices

• Room-temperature ferromagnetic and optical spintronic materials: practical applications.

• Theoretical work: predicted ferromagnetism above room temperature. T. Dietl et. al. Science 287, 1019-22 (2000)

• Very few studies have been done on the ferromagnetic and UV-PL properties of TM-doped ZnO nanoclusters.

B.T.Jonker, NRL

S.D.Sarma

Texture Controlling of FePt:B2O3 Composite Films8

(a) Supersonic nozzle source(b) Gas-aggregation source(c) Laser evaporation source(d) Pulse arc cluster ion source

Nanocluster Sources

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Sputtering-Gas-Aggregation Source

• Deposition rate > 5 Å/s, ~ 10 mg/hour• Ionization rate > 60%, (35% positive charged ion,

30% negative charged ion for Cu clusters), • Controlled monodispersive size range 1 to 100 nm

pump

O2 O2

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History of SGA Cluster Source

1st generation, University of Freiburg, 1995, ~ 0.1 mg/h

2nd generation, University of Nebraska-Lincoln, 2000, < 1 mg/h

3rd generation, University of Idaho, 2003, ~10 mg/h

4th generation, University of Idaho, 2005, testing, ~2 g/h...Nth generation, 20??, up to kg/h.

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•Cluster size: D = 1 to 100 nm•Materials:Co, Fe, Ni, Ag, Al, Cu, Mg, Mo, ZnOSi, Ti, TM-ZnO, TiO2, CoPt, FePt, TiN, TiAlN, Al2O3

Cluster Deposition System

Cluster Deposition System12

13Nanocluster-Assembled Materials

1: “Magnetism of Nanophase Composite Films”,Handbook of Thin Film Materials, Chapter 7. Vol. 5, edited by H.S. Nalwa, Academic Press, 2002.

2: PRB 66, 064404 (2002)

3: Paper in the book of “Cluster and Nanostructure Interfaces”, ed. By P. Jena, S.N. Khanna and B.K. Rao, World Scientific, 2000.

Cluster-Surface Processing14

• K.-H. Meiwes-Broer,“Metal Clusters at Surfaces”,Spring, 2000.

• P. Milani, S. Iannotta, “Cluster Beam Synthesis of Nanostructured Materials”,Springer, 1999.

• Y. Qiang, et al.“Hard coatings (TiN, TixAl1-xN) deposited at room temperature by energetic cluster impact”, Surf. Coat. Technol. 100-101, 27 (1998).

E

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Porous nanofilm of soft landing Fe nanoclusters

FePt nanoclusters embedded in C matrix

10 nm

16

TiN cluster films:a) 0.03 eV/atom, b) 0.5 eV/atomc) 1.0 eV/atom and d) 10 eV/atom

a b

c d

Experimental Set-up

+0-40 kV

+

Cluster Beam Neutral beam

_ ~40% negatively charged,

35% positive charged,

~25% neutral

In the cluster beam measured by a microbalance.

~20 nm negatively charged Fe nanoclusters impact on the surface of Si to form a dense iron thin film.

100 nm

Soft landing Fe nanoclusters on Si

15 keV, Fe nanoclusters on Si MD simulation (1 eV/atom)

Results for 15 KeV

a, b, and c are the ellipsoidal axes of the deposited clusters with a>b>c

b.

b

a

a c

a b c Potenti al (kV) H c

(Oe) H ex (Oe)

H c (Oe)

H ex (Oe)

H c (Oe)

H ex(Oe)

0 78 0 78 0 68 0

5 0.50 0.63 2.06 0.43 9.66 0.45

15 0.65 0.22 2.46 0.46 6.70 1.20

Summary of Magnetic Measurements

1. As small shifting of the hysteresis curve (Hex) was observed for each sample. This is due to the formation of a thin oxide layer over the film of pure Fe cluster films.

2. Magnetic moments: a) 102 emu/g for 0 kV, and b) 201 emu/g for 15 kV.

Synthesis of TM-doped ZnO Nanoclusters

Schematic representation

He, Ar, O2

Cluster formation : Target (Zn-TM), control percentage of TM-dopant » Cluster growth process (He, Ar, O2) » Cluster deposition.

Substrate holder

Cluster sourceDeposition chamber

Sputtering gun

Target (cathode)

Cluster beam

TM

3” Zn target

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Co-doped ZnO clusters

Co doped ZnO nanoclusters

0 200 400 600 800 1000 12000

1

2

3

4

5

6

7

8

ZnLM

M

O1sZn

LMM

ZnLM

M

C1sZn

3sZn3p

ZnLM

M

Zn2p

3

Zn2p

1

Zn2s

Zn3d

c/s

(105 )

Binding energy (eV)

Average crystallite size of ZnO nanocluster film ~ 7nm.

a=3.248, c=5.216ZnO nanoclusters

ZnO bulk

Average crystallite size of 5% Co-doped ZnO ~ 7.5nm.

Identical to bulk ZnO wurtzite

structure

As no dopant elements are observed with XRD analysis, XPS was done on the samples

Co is incorporated into the zinc oxide wurtzite structure.

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0 200 400 600 800 1000 12000

2

4

6

8

10

12

Co

LMM

Co2

p3, C

0 LM

M

ZnLM

M

Zn2s

Zn2p

1Zn2p

3O

KLLO1s

ZnLM

M

ZnLM

MZn

LMM

C1sZn

3sZn3p

Zn3d

c/s

(105 )

2% Co doped ZnO

5% Co doped ZnO

Binding energy (eV)

XPS spectra of 2% and 5% Co doped ZnO nanocluster films,the composition in atomic percent:

1) 2% sample: Co=1.82, O=48.64 and Zn=49.64; 2) 5% sample: Co=5.12, O=48.31 and Zn=46.57.

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XRD

XPS

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MPMS XL 7 System, Quantum Design Inc.

• Magnetic field: 7 T• Temperature: 1.5 – 400 K• Sensitivity: 1 x 10-8 emu

Magnetic Properties

Magnetic properties of TM-doped ZnO nanoclusters

-4500 -3000 -1500 0 1500 3000 4500-0.0003

-0.0002

-0.0001

0.0000

0.0001

0.0002

0.0003

0 100 200 300 400

2.7x10-4

2.8x10-4

2.8x10-4

2.9x10-4

3.0x10-4

Ms

(em

u)

Temp (K)

Mr=8.30E-6 emu

Hc=34.51 Oe

T=400K

MO

MEN

T (e

mu)

FIELD (Oe)

Ms=2.7617E-4 emu

5% Co-doped ZnO

0 50 100 150 200 250 300 350 400

30

60

90

120

150

180

210

Coe

rciv

ity (H

c) O

eTemp (K)

Ti-ZnOCo-ZnO V-ZnO Ni-ZnO

Hysteresis loop is observed for 5% TM-doped ZnO nanoclusters at 400KSaturation magnetization and coercivity decreased with increase in temperatureCoercivity varied with the dopant elements: mechanism?

Ti V Co Ni0

1

2

3

4 5K 300K

Satu

ratio

n M

agne

tizat

ion

(mB/d

opan

t ato

m)

Saturation magnetizations for 4 different dopants at 5K and 300KIsovalent Ti and Co exhibit smaller magnetization compare to the dopants that

exhibit mixed valancy

Different Concentration Different Dopants

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surf 2.5 50

5

10

+4

+4

+5

+4

+4

Depth (nm)

Con

cent

ratio

n (%

)

Ti-ZnO V-ZnO

+4

Depth Profile

452 454 456 458 460 462 464 466

Ti 2p3/2 459.4 eV

c/s

(a.u

)

Binding energy (eV)

Ti +4

770 780 790 800 810

Co +2

Co 2p1/2 797.2 eV

Co 2p3/2 781.4 eV

c/s

(a.u

)

Binding energy (eV)

Nanoclusters deposited on Si wafer before and after Ar+

sputtering

Ti and Co are isovalent before and after Ar+ sputtering

510 512 514 516 518 520 522

After Ar+ sputtering

V2p 516.8 eV

V 2p 517.8 eV

Binding energy (eV)

c/s(

a.u)

V +4

V +5

Before Ar+ sputtering

840 850 860 870 880 890 900

After Ar+ sputtering

Before Ar+ sputtering

Ni +3Ni 2p 857 eV

Ni +2Ni 2p 853.8 eV

c/

s (a

.u)

Binding energy (eV)

V and Ni showed mixed valancy, i.e. the oxidation state altered before and after Ar+ sputtering

Mn3+ 3d4 Mn4+ 3d3

t2g

eg

t2g

eg

Double-exchange interaction

S. Blundell, Magnetism in condensed matter, OXFORD university press

Mn3+ 3d4 Mn4+ 3d3

Double-exchange interaction via carriers can be the reason for enhancedferromagnetism in Ni (V)-doped ZnO nanoclusters

Ferromagnetism is expected in systems where TM atoms with incomplete d-shells do not form the nearest neighbors so that indirect ferromagnetism dominates over the

direct antiferromagnetic coupling in the presence of carriers.

This indirect ferromagnetism by Zener is used to explain the magnetism observed in Ti (Co)-doped ZnO nanoclusters

dxy, dxz, dyz

dz2 d

x2 y2−,

eg

t2g

Co2+

Ni3+Ni2+

3d7 3d8 Ti4+

V5+V4+

eg

t2g

3d0 3d1

Schematic electronic configurations of the dopants in ZnO nanoclusters in the tetrahedral substitution sites

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Double-Exchange

The electronic configurations of V5+, V4+, Ni3+ and Ni2+ are 3d0, 3d1, 3d7 and 3d8, which results in unoccupied or partially occupied spin-up or spin-down levels, allowing the electrons to hop to the 3d level of its neighbor with parallel spin through interactions with p-orbital and thus reducing the kinetic energy through the charge state coupling. Double-exchange interactions due to the mixed valance states [Blundell 2001] via holes can be a reason why Ni (V)-doped ZnO clusters exhibits a higher magnetic moment than the Ti (Co)-doped clusters [Dietl 2000].

If the magnetic moments of the neighboring transition metals are in the same direction, the d band of the transition metal is widened by the hybridization between the spin states introducing carriers in the d band that can lower the band energy in ferromagnetic configuration and hopping is possible.

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αα

αβαβαβα

σ jvvuij

iuuvwijiuiu

uiiHDEH aataaSJH ∑∑

⟩⟨

++− −⋅−=

,,,,,,

r

where 1’st term with JH is the Hunds coupling term between eg and t2g electron and 2’nd term is due to eg electron hopping.

The double-exchange appears only when the spins of the d shells of both Ni2+

(V4+) and Ni3+ (V5+) sites are parallel or one of the hopping site is empty so thatthere is no impedance for the electron transfer.

In the case of Ni (V) doped ZnO, the system is inherently degenerate due to the presence of Ni (V) ions of two charges as explained by Zener 1951.

The origin of ferromagnetism in Ti (Co)-doped ZnO nanoclusters is due to the domination of indirect ferromagnetic coupling via carriers over direct antiferromagnetic coupling.

Ultraviolet Photoluminescence and Raman Lab(Dr. L. Bergman, Physics Department, UI)

Optical Properties

UV-PL SpectraUV-PL Spectra

No visible luminescence

The PL spectra of bulk ZnO vs ZnO nanoclusters.

PL energy peak of bulk ZnO=3.30 eV

PL energy peak of 2% Co-doped ZnO=3.331 eV, at RT

Blue shift in PL energy is observed for pure ZnO nanoclusters and 2% Co-doped ZnO nanoclusters

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ZnO Nanoparticles ZnO Nanoclusters (7 nm in diameter)

Wong and Searson, APL, 74, 2939 (1999)ZnO was fabricated by a colloidal process.

Quantum Confinement Effect at RT

No visible PL~ 125 meV blueshift in VU

UV

Visible

39

2**

22

2 )(26.13

DmmmEE

heregex +

+−=hπ

εμ

Reduced masses: me*=0.32me, and mh*=0.27me. εr =5.8, D is the quantum dot diameter, Eex is the PL excitonic emission as measured via PL.

Calculated result showed a particle size of ~ 7 nm, a prediction which is consistent with the XRD and the TEM studies.

Bandgap energy (3.36 eV),

Binding energy (60 meV),

Confinement effect

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3.1 3.2 3.3 3.4 3.5

Inte

nsity

(A.U

.)

PL Energy (eV)

3.331

FWHM180 meV

ZnO Nanocluster

3.1 3.2 3.3 3.4 3.5

Inte

nsity

(A.U

.)

PL Energy (eV)

3.341

FWHM 196 meV

ZnO Nanocluster(3% Co)

3.05 3.1 3.15 3.2 3.25 3.3 3.35 3.4

Inte

nsity

(A.U

.)

PL Energy (eV)

3.300

FWHM66 meV

ZnO Bulk

UV-PL spectra of bulk ZnO, ZnO nanoclusters and 3% Co-doped ZnO nanoclusters at RT. There is no PL for larger than 5% Co-doped ZnO nanoclusters at RT

• No contamination (UHV)• Perfect crystal (XRD)

41Cathodoluminescence – CNT-FED Nanodevice

Doped ZnO nanoclusters deposited on indium tin oxide (ITO) coated glass substrate and the sample is assembled with a carbon nanotube (CNT) electron emission in a diode mode with 600 microns gap between them. The cathode cross section is ~1 cm2. A bias is applied between anode and cathode: a) 10 mA (1.7 kV) current, and b) 20 mA (1.8 kV).

PL

ITO CNT

Applied Nanotech, Inc.Austin, TX

Nanotechnology, 18 (2007) 295703

42

Cathodoluminescence spectra of ZnO nanoclusters measured using different electron emission currents. The blue curve is from 10 mA CNT electron emission current and the red curve is from the 20 mA.

380

390

Summary and Future Plan• A cluster deposition system has been developed to make

cluster-assembled materials.• TEM, XRD, AFM and XPS have used for studying the

nanoclusters.• Magnetic and optical properties of the nanomaterials made

out of clusters as building blocks have been investigated.• Systematic study regarding the origin of ferromagnetic and

UV-PL properties of Co-doped ZnO clusters and also detail study on the effect of oxygen concentration on these properties.

• 2% , 3% and 5% Co-doped ZnO nanocluster films are ferromagnetic. Only 2% - 3% Co-doped ZnO has ferromagnetic and UV optical emission at RT.

• 5% Co, V, Ni and Ti-doped ZnO nanocluster films are synthesized at room temperature.

• Anomalous double-exchange enhanced ferromagnetism in transition metal-doped ZnO nanoclusters above RT for the dopants that exhibit mixed valance.

• Core-shell nanostructured Zn/ZnO clusters are synthesized and characterized.

• In future we will investigate size effect and dopant concentration dependence of clusters on magnetic and optic behavior.

Undergraduate students:

1. Joe Nutting, physics, UI2. Daniel Meyer, physics, UI3. Michael G. Marino, REU, University of Notre Dame4. Daniel Sikes, REU, North Carolina State University5. Zachary Billey, REU, Linfield College

Graduate Students:

1. Jiji Antony, physics graduate student, 2. Sweta Pendyala, graduate Student of EE department,3. Chandra Kakaraparthi, graduate student of EE department,4. Amit Sharma, physics graduate student5. Muhammard Faheem, physics graduate student.6. Ashewin S. Shenoy, EE department

Postdoc: Dr. Y. F. Xun

Collaboration:

Financial supported by:NSF, DOE, NIHDOE-EPSCoRNSF-EPSCoRBattelle/PNNL

UI:

Dr. L. Bergman, Dr. W. Yeh, PhysicsDr. Ch. Berven

Dr. Y.K. Hong Materials

Dr. R. Crawford MMBBDr. A. Paszczynski

National and International:

Dr. Sumiyama, NU, JapanDr. G.H. Wang, ChinaDr. Sellmayer, UNLDr. Haberland, FreiburgDr. D. Baer, PNNLDr. L-S Wang, WSU/PNNLDr. C-M. Wang, PNNLDr. Engelhard, PNNLDr. M. McKluskey, WSUDr. M. Zhang, UWDr. Y-H. Lin, PNNLDr. P. Tratnyek, OHSU

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Spring Valley LakeMoscow, ID

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