2D Materials – The New Flatland
SS 2014
Ursula Wurstbauer
ZNN 1.004
[email protected]
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 1
Timetable
Nr Date (10-12)
Topic
2
15.04.
Fabrication and Making the Invisible Visible (Microscopy Methods)
22.04.
Osterferien
29.04.
Light Matter Interaction I: Phase Contrast, Ellispometry and Dielectric Constants
06.05.
Studentische Vollversammlung
4
13.05.
Light Matter Interaction II: Phonons in 2D
5
13.05.
12:15-13:00
Peculiarities of Band-Structure in 2D Materials (brown bag lecture - pizza provided)
6
20.05.
Ultrathin Transistor Devices
7
27.05.
2D Material Heterostructures: Optoelectronics
8
03.06.
2D Material Heterostructures: Solar Energy Conversion & Harvesting
10.06.
Pfingstferien
9
17.06.
Transport Properties and Quantum Phenomena in Graphene
10
24.06.
Spin- and Valleytronic in 2D Materials
11
01.07.
Topological Insulator - the conducting surface
12
10.07.
Recent progress & 2D Materials at TUM and time for questions
lab tour subsequent to lecture for interested participants
3
2D Materials SS 14
Fundamental
& Future
Introduction and Material Overview
Devices
08.04.
Technology
1
08. 2D Heterostructures: Solar Energy Conversion 2
Last Week …
Optoelectronic Devices
In general electronic devices that:
• detect
• generate
• control
• interact with
light
(usually in visible and IR regime)
Some examples:
Photodiode
Solar Cells
Photoelectric effect
Phototransistor
(Photovoltaic effect)
Photoamplifier
Photoresistor
Switches
Photoconductivity
Charge-Coupled Imaging Devices (e.g.CCDs)
Light Emitting Diodes (LEDs)
(Stimulated) Emission
Laser Diodes
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 3
Involved Optical Processes:
Reflection (I0R)
Propagation
2
n~  1
R ~
n 1
Luminescence
Transmission
T=(1-R1)e-at(1-R2)
Incident
Beam (I0)
Scattering
Air
(n=1)
Air
(n=1)
(Raman, Rayleigh...)
I=I0e-Nsz
Medium
~)
(n= n
Absorption of MoS2
t
K. F. Mak, et al. Phys. Rev Lett. 105, 136805 (2010).
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 4
Interband Transitions in a nutshell
Interband absorption
• Electrons promoted from valence band to conduction band by
absorbing a photon  electron-hole pair generation
• Possible for hwph > Eg
• Optical transitions are “vertical” on an E-k diagram
Transition rate (absorption coefficient)
• Transition rate 
|pcv|2 and JDOS (g(hwph))
Momentum matrix element of central
cell parts of the solid wavefunction
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 5
Examples for 2D Material based optoelectronic devices
1. Ultrasensitive TMDC Photodetector
Photoresponse
Enhancement-mode transistor:
Increasing conductivity with increasing light intensity
due to absorption of e-h pairs in direct SC.
5µm
O. Lopez-Sanchez et al. Nature Nanotech. 8, 497 (2013)
2. Contributions to Photocurrent in MoS2
Iph pA
Photocurrent
Photovoltaic and thermoelectric
contributions to photocurrent
2D Materials SS 14
M. Hoheneder, E. Parzinger et al. (2014)
08. 2D Heterostructures: Solar Energy Conversion 6
3. Ultrafast Graphene Devices
F. Xia, T. Mueller, Y. Lin, A. Valdes-Garcia, P. Avouris, Nature Nanotechnology 4, 839 (2009).
T. Mueller, F. Xia, P. Avouris, Nature Photonics 4, 297 (2010).
Fast response
Displacement currentdensity

E
jD   0
t
FWHM = 4.4 ± 0.2 ps
UThermo = (SGraphene – SStripline) ∙DT,
L. Prechtel, et al. Nature Comm. 3, 646 (2012).
2D Materials SS 14
t
thermoelectric
= 130 ± 10 ps
08. 2D Heterostructures: Solar Energy Conversion 7
2D Materials – The New Flatland
Chapter 8
2D Material Heterostructures: Solar
Energy Conversion & Harvesting
Ursula Wurstbauer
ZNN 1.004
Tel: 289 11445
[email protected]
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 8
Today’s Lecture
Solar Energy Conversion
• General requirements
Solar Hydrogen Production by Splitting of Water
• Requirements
• Principles
• Potential for 2D Materials
Initial Examples:
Photocatalytic Hydrogen Evolution from 2D Materials
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 9
Solar Energy
http://www.exergypower.com.au/SolarPower.html
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 10
Power Generation in Germany (January – April 2014)
30%
nuclear
power
brown
coal
hard
coal
gas
wind
solar
biomass
hydroelectric
fossile fuels
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 11
Solar energy conversion - PV
Requirements:
Sunlight
absorption
Separation
of e-h pairs
Transport of
electricity
2D Materials SS 14
Transport of
e- & h+ to
electrodes
Storage of
electricity
08. 2D Heterostructures: Solar Energy Conversion 12
Transport of
electricity
• High losses
Storage of
electricity
a) Batteries, Super-capacitors:
• Expensive to built
o Expensive
• ‚Other issues‘
o Limited storage density
• No electricity on demand:
o Environmental issues
no power generation at
b) Chemical bonds:
o night
• Solar fuels e.g. (H2, Methanol, ...)
o cloudy, rainy days
→ Artificial photosynthesis
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 13
Photocatalytic Water Splitting
… artificial photosynthesis
Requirements for (efficient) photocatalyst:
1. High photo-absorption efficiency in visible regime
requirements
for solar cell
(PV device)
2. Separation of e-h pairs
3. Appropriate charge carrier mobility
4. High stability in water (also under illumination!)
5. Bandgap Egap>1.23eV (energy needed for
catalytic process)
Additional
requirements
6. Band edge suitable for H2O redox potential
7. (High earth abundance)
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 14
1. Absorption efficiency
Comparison of ‘traditional’ and new PV materials
Standard Solar Spectra
visible regime
(Bernardi et al., Nano Lett. 2013, 13, 3664)
TMD monolayers have high
absorbance of 5-10% in visible
(thickness of only 6.5 Å!)
15 nm of GaAs or 50 nm of Si are
needed to absorb the same fraction
of sunlight as a TMD monolayer
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 15
2. Separation of e-h pairs
• microscopic process of charge carrier generation & separation
• for separation in-plane electric field (built-in) required
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 16
3 Examples to introduce built-in electric field
1. Schottky barrier using MoS2/graphene
3.3 Å
6.5 Å
M1/M2: low/high workfunction metals
Schottky barrier for holes (p-SB)
2. 2D Material Heterojunction
MoS2
WS2
Type-II heterojunction
at WS2/MoS2 interface
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 17
3. MoS2 multilayer Schottky junction using different electrodes
Device Scheme
Scheme of Band structure
I-V characteristics
• 2.5 % of incident laser power extracted into electrical power
• Higher power conversion efficiency expected for 1L-MoS2
Fontana et al., Sci. Rep. 3, 1634 (2013)
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 18
Efficiency of solar cells
Power conversion efficiency:
PCE 
PMPP
I
VMPP
IMPP
ISC
VOC
V
I V  FF
PMPP
 SC OC
mW
Pincident
100 2
cm
FF = PMPP/(UOCxIOC) fill factor: 0.3 - 0.6
Isc = 0.7 x Iabs with 0.7 for internal quantum
efficiency
VOC depending on system
SC: Short circuit
OC: Open Circuit
MPP: Maximal Power Point
Comparison with record ultrathin solar cells
Power density values higher than any known
energy generation and conversion device
2D Materials SS 14
Bernardi et al., Nano Lett. 13, 3664 (2013)
08. 2D Heterostructures: Solar Energy Conversion 19
3. Charge carrier mobility
Egap
Graphene
Graphene
nanoribbon
(~10nm)
0
< 200 meV
mobility (RT)
>15,000 cm2/(Vs)
<200 cm2/(Vs)
MoS2 - ML
1.9 eV
>200 cm2/(Vs) (n-type)
WSe2 - ML
1.5 eV
>250 cm2/(Vs) (p-type)
Si-MOSFET
1.1 eV
electrons: <1,500 cm2/(Vs)
holes:
~ 450 cm2/(Vs)
GaAs
2D Materials SS 14
1.4 eV
electrons: ~8,500 cm2/(Vs)
holes:
~ 400 cm2/(Vs)
08. 2D Heterostructures: Solar Energy Conversion 20
4. Stability in water
ω > Egap
ω < Egap
t > 3h,
P  3mW
t > 10s,
P ≥ 0.5mW
ω > Egap
ω > Egap
ω < Egap
2D Materials SS 14
t > 10s,
P ≥ 0.5mW
ω > Egap
pristine MoS2:
highly stable under
illumination in
water (no
photocorosion!)
MoS2 edges:
water assisted
laser cutting for ω
>Egap
defected MoS2:
water assisted
laser cutting for ω
>Egap
08. 2D Heterostructures: Solar Energy Conversion 21
5. Bandgap Egap > 1.23eV
Solar spectrum
MoS2
2D Materials SS 14
WSe2
Monolayer
MoS2: Eg = 1.9 eV
WSe2: Eg = 1.5 eV
08. 2D Heterostructures: Solar Energy Conversion 22
6. Band edge for H2O redox potential
Solar Water Splitting – Principle
e-
CB
H+
e-
H2
ħ
h+
VB
Semiconductor:
Oxidation:
Reduction:
Overall:
2D Materials SS 14
h+
H2O
½O2+2H+
2ħ  2e- + 2h+
H2O + 2h+  2H+ + ½O2
2H+ + 2e-  H2
H2O + 2ħ  H2 + ½O2
08. 2D Heterostructures: Solar Energy Conversion 23
Typical materials
stability issue
issue with work function
RHE
0
MoS2
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 24
Overview water splitting concepts
(on example of n-type photoanode watersplitting device)
Energetic requirements
Involved Processes
•
Photon irradiation & absorption
•
Thermodynamic energy to split water
•
e-h pair creation & separation
•
Catalytic overpotential for
•
Charge transport
Hydrogene evolution reaction (HER)
•
Interfacial reactions
Oxygen evolution reaction (OER)
Z. Chen, H. N. Dinh, E. Miller, Photoelectrochemical Water Splitting, Springer (2013)
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 25
Some numbers
• Total solar irradiation per year:
5.4 1024 J
• World energy consumption 2012:
5.9 1020 J
1h of sun irradiation enough
to cover world energy
consumption in a year!
• Solar energy “for free”
To provide 1/3 of the projected energy
need in 2050 from solar energy:
• 10 000 solar plants (5km x 5km) with
solar energy conversion efficiency of
10%
• Required area would be 1% of earth
desert area
• Production per day:
570 tons of H2 gas (AM1.5G
irradiation)
K. Maeda et al. J. Phys. Chem. Lett. 1, 2655 (2010)
2D Materials SS 14
Y. Tashibana et al. Nature Photonics 6, 511 (2012)
08. 2D Heterostructures: Solar Energy Conversion 26
MoS2 as efficient photocatalyst
Requirements:
1. High photo-absorption efficiency in visible regime
2. Separation of e-h pairs
3. Appropriate charge carrier mobility
4. High stability in water (also under illumination!)
5. Bandgap Egap>1.23eV (energy needed for
catalytic process)
6. Band edge suitable for H2O redox potential
7. (High earth abundance)
Research on MoS2 as photocatalyst
still in its infancy…
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 27
MoS2 as photocatalyst: initial reports
1. Layer-dependent Electrocatalysis of MoS2 for Hydrogen Evolution
Current Density Measurements
•
•
•
•
Tafel Plot
Layer dependent electrocatalysis
1L most effective for Monolayer MoS2
Layer dependent correlated with hopping of electrons in vertical direction
Role of edge sites (active sites for catalytic process!)
2D Materials SS 14
Y. Yu et al. Nano Lett. 14, 553 (2014)
08. 2D Heterostructures: Solar Energy Conversion 28
Electrochemical Measurements
Andreas Reitinger, WSI (2014)
WE: Working Electrode - HER
CE: Counter Electrode - OER
RE: Reference Electrode (in electrolyte)
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 29
2. Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers
→ role of edge sites for hydrogene evolution reaction
Fabrication and crystalline structure vertically alligned layers (CVD process)
Possible Edge Sites
TEM image
D. Kong et al. Nano Lett. 13, 1341 (2013)
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 30
Electrochemical Measurements
Polarization curves
Tafel Plot
HER activity
stability test
for MoS2
stability test
for MoSe2
Catalyst stable
in water
Edges are chemically active because of dangling bonds
D. Kong et al. Nano Lett. 13, 1341 (2013)
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 31
Summary
Chapter 8: 2D Material Heterostructures: Solar Energy Conversion & Harvesting
• General requirements for solar energy conversion (absorption, e-h
separation, transport)
• Storage of energy in chemical bonds: solar water splitting
Requirements: excellent PV properties in visible regime,
thermodynamic potential for redox, stability in water, earth abundance
• TMDCs potential as photocatalyst for solar hydrogen production
Further readings:
M. Bernadi et al., Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using TwoDimensional Monolayer Materials, Nano Lett. 13, 3664 (2013).
Y. Yu et al., Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution, Nano Lett. 14, 553 2014).
D. Kong, et al., Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers, Nano Lett. 13, 1341
(2013).
J. A. Turner, A Realizable Renewable Energy Future, Science 285, 687 (1999).
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 32
Timetable
Nr Date (10-12)
Topic
2
15.04.
Fabrication and Making the Invisible Visible (Microscopy Methods)
22.04.
Osterferien
29.04.
Light Matter Interaction I: Phase Contrast, Ellispometry and Dielectric Constants
06.05.
Studentische Vollversammlung
4
13.05.
Light Matter Interaction II: Phonons in 2D
5
13.05.
12:15-13:00
Peculiarities of Band-Structure in 2D Materials (brown bag lecture - pizza provided)
6
20.05.
Ultrathin Transistor Devices
7
27.05.
2D Material Heterostructures: Optoelectronics
8
03.06.
2D Material Heterostructures: Solar Energy Conversion & Harvesting
10.06.
Pfingstferien
9
17.06.
Transport Properties and Quantum Phenomena in Graphene
10
24.06.
Spin- and Valleytronic in 2D Materials
11
01.07.
Topological Insulator - the conducting surface
12
10.07.
Recent progress & 2D Materials at TUM and time for questions
lab tour subsequent to lecture for interested participants
3
2D Materials SS 14
Fundamental
& Future
Introduction and Material Overview
Devices
08.04.
Technology
1
08. 2D Heterostructures: Solar Energy Conversion 33
2D Materials – The New Flatland
Chapter 9
Transport Properties and Quantum
Phenomena in Graphene
Ursula Wurstbauer
ZNN 1.004
Tel: 289 11445
[email protected]
2D Materials SS 14
08. 2D Heterostructures: Solar Energy Conversion 34