20191028 Wang Seminar at NCTU (Near-field Thermal Radiation)

Near-field Thermal Radiation for
Thermal Energy Conversion and Heat Modulation
Research Seminar at National Chiao Tung University
Hsinchu, Taiwan, October 29, 2019
Liping Wang, Ph.D.
Associate Professor in Mechanical and Aerospace Engineering
Director of Nano-Engineered Thermal Radiation Laboratory
School for Engineering of Matter, Transport & Energy
Arizona State University, Tempe, AZ USA
http://faculty.engineering.asu.edu/lpwang
Email: liping.wang@asu.edu

2
Nano-Engineered Thermal Radiation Group
Phoenix, AZ
² 5th largest city in US with population ~1.7 million (2018)
² Abundant solar radiation: ~300 days sunshine /year
² Average temp.: summer >35degC, winter ~10degC
² < 5 hr driving to Los Angeles, San Diego, Las Vegas
² 5 hr driving to Grand Canyon and other national parks
² 2 hr driving to Flagstaff AZ (Ski resort, Indian habitat)
² ASU is Located in Tempe, Arizona, USA, southeast in the
Phoenix Metro Area with Intel, Freescale, etc
o Phoenix, AZ
ASU

3
ASU’s strong commitment to solar energy
Total Solar Generation Capacity: 24.1 MW (50% ASU daytime peak load)
(PV: 21.8 MW; Solar thermal: 13,908 MMBTUs = 2.3 MW equivalent)
Total Solar Systems: 89
Total Number of PV Panels Installed: 81,424
Total Number of CPV Modules Installed: 8,652
Total Number of Solar Collectors Installed: 9,280
Total Number of Shaded Parking Spaces: 5,952
http://about.asu.edu, https://cfo.asu.edu/solar
² Established in 1885
² #1 in the US for Innovation by US News & World Reports
(#2 Stanford, #3 MIT)
² #1 student enrollment (~100,000) in US universities (2018)
² #1 public university chosen by international students
² Top 1% of world’s most prestigious universities
² Top 10 in total research expenditures among US universities
without medical school
² #42 Graduate program in Engineering by US News
² One of the greenest universities in USA
ISTB4
May 2012
300K sq. ft
Old Main
built in 1898
A New American University

4

5
Outline
1. Background
• Near-field thermal radiation and its potential applications
• Literature review – theoretical studies by Wang group
• Literature review – recent experiments by other groups
2. Super-Planckian Near-field Thermal Radiation Measurements
• Al thin films with nanoparticles as vacuum gap spacers
• Heavily-doped Si with polymer posts and in-situ gap determination
3. Gate-tunable Near-field Radiative Heat Flux with Graphene
• Graphene characterization
• Theoretical prediction
• Preliminary experimental results
4. Future Work and Acknowledgments

far field vs. near field
2
T
12
q¢¢
1
T
d
l evanescent wave
2
T
12
q¢¢
1
T
d
Subject to blackbody radiation
4 4
12 1 2
( )
q T T
es
¢¢ = -
Far field (d >> l):
Coupling of evanescent waves
Near field (d < l):
Near-field heat transfer
enhancement
1. Background
6
Liu, Wang and Zhang, Nanosc. Microsc. Therm. 19, 98 (2015)
What is near-field thermal radiation?

7
Nano-Engineered Thermal Radiation Group Liu, Wang and Zhang, Nanosc. Microsc. Therm. 19, 98 (2015)
Near-field radiation applications
1. Background

8
Near-field thermophotovoltaic energy conversion (by Wang Group)
Metallodielectric
Near-Field TPV Emitter
Near-field TPV cell
with Back Reflector
Tungsten Nanowire-
based HMM Near-
field TPV Emitter
Chang, Yang, and Wang*,
IJHMT 87, 237 (2015)
Bright, Wang, and Zhang,
J. Heat. Transfer. 134, 062701(2014)
Yang, Chang, and Wang*,
JHT 139, 052701 (2015)
Nanostructured MM
Near-Field TPV Emitter
Sabbaghi, Yang, and Wang*,
JQSRT (2019)
1. Background

5
10
15
20
330 335 340 345 350
Insulating VO
2
Metallic VO
2
VO
2
Emitter Temperature, T
H
(K)
Net
Heat
Flux,
q"
(kW/m
2
)
d = 50 nm
T
L
= 300 K
"ON"
"OFF" 10
3
10
4
10
5
10
6
0.5
1
1.5
2
10
1
10
2
10
3
forward heat flux
reverse heat flux
rectification factor
Net
Heat
Flux,
q''
(W/m
2
)
Rectification
Factor,
R
Vacuum Gap, d (nm)
Near-field Radiative Heat Flux
Near-Field
Thermal Modulator
Near-field
Thermal Switch
Yang, Basu, and Wang,
APL 103, 163101 (2013)
Yang and Wang,
JQSRT 197, 65-75 (2017)
Yang, Basu and Wang,
JQSRT 158, 69 (2015)
Near-field
Thermal Rectifier
Near-field radiative heat control (by Wang group)
1. Background
9

Nano-Engineered Thermal Radiation Group 10
• NFR between doped Si of 1×1 cm2
• d = 200 to 780 nm, DT= 2 to 30 K
• Using SiO2 posts
• 11 times as high as blackbody limit
• Bow reduction is done by depositing silicon
dioxide at 300 K
• Using FTIR to measure the reflectance to quantify
the gap spacing
Watjen et al. Applied Physics Letters, 2016
Bernardi et al. Nature Communications, 2016
• NFR between two glass surfaces
• d = 1 µm, Th= 323 K, Tc= 297 K
• Small polystyrene microsphere (80 particles)
• Exceeding the blackbody as the gap decreases
below 5 µm
Hu et al. Applied Physics Letters, 2008
• Radiative heat flux between two 5×5 mm2
Si surfaces;
• d = 150 nm and DTmax= 120 K;
• Using SU-8 with a 3.5 µm height and SiO2
stopper with a 150 nm height;
• Calibrated masses ranging from 0.9 to 5g;
• Exceeding the blackbody by 8.4 times;
• Enhancement only due frustrated modes.
Recent experimental NFR
1. Background

• Radiative heat flux between 5×5 mm2
quartz plates
• Minimum gap of 200 nm
• Maximum temperature difference of
156 K
• Exceeding the blackbody by 40 times
at d = 200 nm
Ghashami et al., Phys. Rev. Lett. 2018
• Radiative heat flux between metallo-
dielectric (MD) multilayers (Ti / MgF2)
at nanoscale gaps
• Surface plasmon polaritons supported
at multiple metal-dielectric interfaces
• Minimum gap of 160 nm
• Exceeding the blackbody by 7 times at
d = 160 nm
Lim et al., Nature Commun. 2018
Fiorino et al., Nano Letters, 2018
• NFR between planar silica surfaces
• Separated by gaps as small as 25 nm
• 1200-fold enhancement with respect
to far field
• 700-fold enhancement over black
body limit
1. Background

Shi et al.,
Nano Letters
2015
Hyperbolic
Metamaterials
NFR in nm and
sub-nm gaps
Cui et al.,
Nature Commun.
2016
1. Background

Al thin films – setup with PS nanoparticles as spacer
Sabbaghi et al., PR Applied, in revision (available on arXiv) 13

Al thin films – gap fitting with Si measurement
Sabbaghi et al., PR Applied, in revision (available on arXiv)
14

Al thin films – comparison between measurement and theory
15

Al thin films – near field enhancement over far field
16

Heavily doped Si – vacuum gap created by patterned SU-8 posts
Wafer pre-treatment
SU-8 coating
Post-baking
Posts pattern
exposure
Height calibration
pattern exposure
Pre-baking
Post-baking
Same
Spin
Speed
Microscopic image
of SU-8 post
Thickness of SU-8
under profilometer
Wafer pre-treatment
SU-8 coating
Post-baking
Posts pattern
exposure
Height calibration
pattern exposure
Pre-baking
Post-baking
Same
Spin
Speed
Microscopic image
of SU-8 post
Thickness of SU-8
under profilometer
Anisotropic plasma
dry etching
Anisotropic plasma
dry etching
Ying et al., ACS Photonics, in revision (available on arXiv)
17

Heavily doped Si – thermal measurement setup
18

Heavily doped Si – vacuum gap determined by capacitance method
19

Heavily doped Si – measured near-field radiative flux
20

C-V Measurement
Purchased from Graphenea.com
• Graphene: monolayer, transferred from CVD
• SiO2: 300 nm thermal oxide (ed = 3.8)
• Doped Si: r<0.005 ohm·cm
• Size: 1 cm x 1 cm
https://www.graphenea.com/
𝐶! =
"!""#
$!
=11.2 nF
Electrically gating graphene

Vg < Vcnp
Vg = Vcnp
Vg > Vcnp
Ω
R – V Measurement
Determination of charge neutrality point (CNP)
Vcnp = 90 V

(1-R)
/
(1-R
cnp
)
Measurement Modeling
(1-R)
/
(1-R
cnp
)
Graphene: 0.345 nm
SiO2: 300 nm
Doped Si: 525 um
FTIR measurement with electrical gating
• Spectral reflectance is measured due to opaqueness
• Lowest absorption is observed at Vg = Vcnp = 80V, or µ = 0
• Absorption increased when Vg is away from Vcnp
• Consistent with modeling
|Vg – VCNP|
80V
0V

Fermi level vs. Gating voltage
𝜇 = ℎ𝑣% ⁄
𝜋𝐶! 𝑉
! − 𝑉
&'( (𝐴𝑒)
where vF = 1.1 x 106 m/s
Falkovsky et al., J. Phys. 129, 012004 (2008)
G G G 0
/ ( )
i t
e s we
=
σG
= f (µ,T,ω),
Electrical Permittivity:
where
Predicted gating voltage (Vg) effect on graphene properties

( ) ( ) ( )
1 2
2
0 0 0
1
( ) , , ,
4
w w w w w bx w b b
p
¥ ¥ ¥
¢¢ = = Q -Q
é ù
ë û
ò ò ò
q q d d T T d
( )
( )( ) ( )( )
( )
( ) ( )
( ) ( )
0 0
0 0
0 0
2 2 2 2
012 034 012 034
2 2
2 2
012 034 012 034
2Im 2Im
012 034 012 034
2 2
2 2
012 034 012 034
,
exp( / ) 1
1 1 1 1
( , )
1 1
4Im( )Im 4Im( )Im
,
1 1
B
s s p p
prop
i d i d
s s p p
d d
s s p p
evan
i d i d
s s p p
T
k T
r r r r
r r e r r e
r r e r r e
r r e r r e
g g
g g
g g
w
w
w
x w b
x w b
- -
Q =
-
- - - -
= +
- -
= +
- -
!
!
ì
ï
ï
ï
ï
ï
í
ï
ï
ï
ï
ï
î
2 2
2 2
2
02 12
012 2
02 12
1
i t
i t
r r e
R
r r e
g
g
+
=
+
2 2
2 2
2
02 12
012 2
02 12
1
i t
i t
t t e
T
r r e
g
g
=
+
( ) ( )
( ) ( )
2 2
2 2
2 0 2 0 0
20
2 0 2 0 0
2 0 0 2
20
2 0 0 2
/
/
p p
p
p p
s s
s
s s
r
r
e g g s g g we
e g g s g g we
g g µ s w
g g µ s w
ì - +
ï =
+ +
ï
í
ï - -
=
ï
+ +
î
SiO2
Doped Si
Doped Si
1
2
3
4
d
0
t2
t3
k
β
γ
NFR calculation for graphene
SiO2
where
Thin-film optics with surface currents:
where
• Graphene modelled
as surface current
25

emitter receiver
Predicted NFR heat flux without gating (Vg = 0)
Heat
flux,
q
(W/m
2
)
Vacuum distance d = 200 nm
Vcnp = 80 V
qBB = 1 kW/m2
for ∆T = 100∘C

27
80 V
0 V
80 V
0 V
80 V
0 V
200 nm
350 nm
500 nm
• Greater tuning at smaller gap distance
Predicted NFR heat flux modulation with gating
Doped Si
Doped Si
Emitter at T1
Graphene
(ungated)
Graphene
(gated w/ Vg)
SiO2
SiO2
Receiver at T2
|Vg – VCNP|
Heat
flux,
q
(W/m
2
)
Vcnp = 80 V
qBB = 1 kW/m2
for ∆T = 100∘C

0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Capacitance
(nF)
Time (a.u.)
d = 350 nm
Cgap1 = 0.4 nF
Cgap2 = 1.35 nF
0 20 40 60 80 100
2480
2500
2520
2540
2560
2580
2600
Heat
flux
q
(W/m
2
)
Time (a.u.)
Theoretical:
2224 W/m2
Measured NFR heat flux without gating (Vg = 0) between graphene
Doped Si
Doped Si
SiO2
SiO2
Graphene
(ungated)
Graphene
(ungated)
Preliminary
Results
∆T = 45∘C

Ongoing and Future Work
Plate-Plate tunable NFR devices
graphene, VO2, etc
d > 100 nm
Tip-surface NFR
2D materials
d ~ nm
Sphere-plate NFR
metasurfaces, 2D materials
d > 30 nm
(DURIP Award)
29

30
Acknowledgements
Team Players:
o Dr. Yue Yang (2016-2012, currently Assistant Professor at Harbin Institute of Technology, Shenzhen)
o Dr. Hao Wang (2016-2012, currently postdoc at Lawrence Berkeley National Lab)
o Dr. Jui-Yung Chang (2017-2012, currently Assistant Professor at National Chiao Tung Univ., Taiwan)
o Dr. Hassan Alshehri (2018 – 2014, currently Assistant Professor at King Saud University, Saudi Arab)
o Dr. Linshuang Long (3rd year postdoc, VO2 metamaterial and graphene metasurface)
o Dr. Qing Ni (3rd year postdoc, TPV measurement and ultrathin cells)
o Mr. Payam Sabbaghi (5th year PhD student, near-field TPV and near-field radiation measurement)
o Ms. Sydney Taylor (4th year PhD student, NASA NSTRF Fellow, VO2 fab and metafilm)
o Ms. Xiaoyan Ying (3rd year PhD student, near-field radiation measurement and 2D materials)
o MS and UG students: Ramteja Kondakindi, Ryan McBurney, Lee Lambert, Niko Vlastos, etc

20191028 Wang Seminar at NCTU (Near-field Thermal Radiation)

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