ANHARMONIC PHONON COUPLING AND IN-PLANE

HETEROJUNCTION FABRICATION IN 2D IN2SE3

John Igo

Collaborators

Dr. Feng Zhao

Dr. Shengwen Zhou

Dr. Zhi-Gang Yu

Matthew Gabel

Committee

Dr. Yi Gu

Dr. Matt McCluskey

Dr. Feng Zhao

Slides

van der Waals (vdW) Materials

Graphene MoS2

• Layered structure

• In-plane covalent bonds

• Inter-layer van der Waals bond

M. M

cC

arthy/N

ISE N

etw

ork

Radisavljevic

et a

l. D

OI: 10.1038/nnano.201

0.2

79

vdW In2Se

3

Tao et a

l. D

OI: 10.1021/nl4008

88p

• One way transformations with increasing temperature• α and β have very similar crystals ∆a/a ~ .9% and ∆c/c ~ 1.8%

473 K 623 K ~ 923 K

Raman Spectroscopy

Crystals

only support

special

vibrational

modes

Stokes

𝜐 = 𝜐𝑖 − 𝜐𝑐

Anti Stokes

𝜐 = 𝜐𝑖 + 𝜐𝑐

Em

ber et a

l, D

OI: 10.1038/s41536

-0

17

-0

01

4-3

Photoluminescence Spectroscopy

• Excite with photons whose

energy is larger than the band

gap.

• Measure the decay from the

bottom of the conduction

band to defect states or to

the top of the valence band

In2Se

3optical properties

Raman Photoluminescence

a

b b

a

b

Band Diagram

• Alpha and Beta are indirect band gap n type semiconductors

• Alpha has a direct band gap at 1.32 eV

a

In2Se

3electrical properties

Wang et a

l, D

OI: 10.1

021/acs.jpclett.7b0108

9 a

b

• Alpha is a n-type semiconductor

• Beta is a n-type semimetal (metal)

Motivation Vertical In2Se

3a/b junctions

• b to a short circuit photocurrent indicates that PTE is dominant

• Thermoelectric Figure of Merit ~ 1 at room temperature

Wang et a

l. D

OI: 10.1

021/acs.jpclett.7b0108

9

Reflectivity Photocurrent

Raman vs Temp

a

b

Model

𝛤 = 𝛤0 + 𝐴[1 +

𝑖=1

𝑛

ሿ𝑛 𝜔𝑖 , 𝑇

For Alpha

105 𝑐𝑚−1 = 20 𝑐𝑚−1 + 85 𝑐𝑚−1

For Beta

109𝑐𝑚−1 = 25𝑐𝑚−1 + 30𝑐𝑚−1 + 54𝑐𝑚−1

Menendez et a

l, D

OI: 10.1103/PhysRevB.29.2

05

1

Curve Fitting

• Extrapolate from lower temperatures

• Iteratively fit individual peaks until,

Fit converges sufficiently

We know fit will not converge

a and b FWHM vs Temp

αβ

Model

𝛤 = 𝛤0 + 𝐴[1 +

𝑖=1

𝑛

ሿ𝑛 𝜔𝑖 , 𝑇

For Alpha

𝛤0 = .51 𝑐𝑚−1

𝐴 ≅ .17 𝑐𝑚−1

For Beta

𝛤0 = .06 𝑐𝑚−1

𝐴 ≅ .60 𝑐𝑚−1

Experiment to find thermal conductivity

Simulation

Experiment

• Extract peak shift vs laser power

• Extract peak shift vs ambient temperature

• Numerically simulate heat dissipation

Peak Position vs Temp and vs Power

𝜒𝑝 ≅ −1.16 ± .07 ∗ 10−3𝑐 Τ𝑚−1 𝑊 𝜒𝑇 ≅ −4.6 ± .16 ∗ 10

−3𝑐 Τ𝑚−1 𝐾

Mechanisms limiting thermal transport

Callaway Model

𝑘 𝑇 =𝑘𝐵

2 𝜋2𝐶

𝑘𝐵𝑇

ħ

3

න0

Τ𝜃𝐷 𝑇

𝜏𝑐𝑥4𝑒𝑥

𝑒𝑥 − 1 2𝑑𝑥

𝜏𝑐−1 =

𝐶

𝑡+ 𝐴𝜔4 + 𝐵𝜔2𝑇 𝑒 Τ−𝜃𝐷 3𝑇

For Alpha

𝐴 ≅ 6.0 ∗ 10−48𝑠3, 𝐵 ≅ 7.8 ∗ 10−19𝑠/𝐾

For Beta

𝐴 ≅ 7.1 ∗ 10−48𝑠3, 𝐵 ≅ 3.6 ∗ 10−18𝑠/𝐾

Callaw

ay D

OI: 10.1103/PhysRev.11

3.10

46

Umklapp Scattering and Anharmonicity

Normal Scattering Umklapp Scattering

• Requires anharmonic potential

𝑈 = 𝑈𝑒𝑞 + 𝑈ℎ𝑎𝑟𝑚𝑜𝑛𝑖𝑐 + 𝑈𝑎𝑛ℎ𝑎𝑟𝑚𝑜𝑛𝑖𝑐• Momentum is not conserved

• Required to explain transfer processes

Ashkroft

& M

erm

in

chapter 2

5

Mechanisms limiting thermal transport

Callaway Model

𝑘 𝑇 =𝑘𝐵

2 𝜋2𝐶

𝑘𝐵𝑇

ħ

3

න0

Τ𝜃𝐷 𝑇

𝜏𝑐𝑥4𝑒𝑥

𝑒𝑥 − 1 2𝑑𝑥

𝜏𝑐−1 =

𝐶

𝑡+ 𝐴𝜔4 + 𝐵𝜔2𝑇 𝑒 Τ−𝜃𝐷 3𝑇

For Alpha

𝐴 ≅ 6.0 ∗ 10−48𝑠3, 𝐵 ≅ 7.8 ∗ 10−19𝑠/𝐾

For Beta

𝐴 ≅ 7.1 ∗ 10−48𝑠3, 𝐵 ≅ 3.6 ∗ 10−18𝑠/𝐾

Callaw

ay D

OI: 10.110

3/PhysRev.11

3.10

46

Novoselov

et a

l., Science

35

3, 461 (201

6)

Gong et al. nature m

aterials

13, (2014)

Kang et a

l., A

pplied

Physics

Letters. 1

02

, 012

11

1 (2

01

3)

Pant et al., N

anoscale

8, 3870 (2016)

• Semiconductor-Semiconductor heterojunctions enable devices

─ Lasers, LEDs, solar cells, etc.

vdW Heterojunctions

Kim

et al. D

OI:10.108

8/2053

-1583/aa5b0e

Laser thinning of MoTe2

• Laser driven post patterning in MoTe2

• Scalable created atomically thin regions

Laser heating in MoTe2

Seo

et a

l. D

OI: 10.1038/s41928

-01

8-01

29

-6

• Local heating of pre-patterned electrodes

• Defects were created (vacancies) causing local

p-type doping

Fabricating in-plane junctions in In2Se

3

• Raise temperature to Tc-100 K

• Patterned using 3-5 mW pulses• Checked using 3-5 μW pulses

ab

β dots in α crystals

• Freely pattern beta dots

• Smooth Topography

• Single Crystallinity

α/β junctions, AFM characterization

• Smooth topography

• ~300mV change in the work function

• ~500 nm junction

KPFM Mapping KPFM and AFM

• PL confirms KPFM

• Strong Short Circuit

Photo Current

a

b

α/β junctions, optical characterization

Electrical Properties of In2Se

3 Lateral Junctions

T = 300K

• Rectifying IV curve

• Non standard diode behavior V0 power law

V > 0, Power law

• V < 0 ~Constant E and n

• Activation Energy

simulation ~.2 eV

experiment ~ .14 eV

Simulation results

Conclusion

We have determined the thermal transport limiting mechanics in a and

b In2Se

3, showing b to possibly be an efficient thermoelectric material.

We have developed a scalable laser-writing procedure capable of

directly defining in-plane phase patterns in 2D In2Se

3layers based

on local optically activated solid-solid phase transitions. These phase

heterojunctions exhibit sharp junctions, and rectifying behavior.

Acknowledgments

Collaborators

Dr. Feng Zhao

Dr. Shengwen Zhou

Dr. Zhi-Gang Yu

Matthew Gabel

Committee

Dr. Yi Gu

Dr. Matt McCluskey

Dr. Feng Zhao

Questions

Questions

SCLC generally shows up as I ~ V2

, why does your device

deviate from that?

Do you need the hBN to shield your In2Se3 ?

Does this junction fabrication transformation have any substrate

dependence?

Does the large depletion region lead to a high quantum efficiency?

What is the minimum size of the b In2Se3 that you can pattern?