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‫ אילן ריס‬.‫ פרופ‬:‫דו"ח סגל מחקר‬
18/3/2014
What is in common to modern memory elements and energy conversion?
Outline:
1. Introduction, what kind of problems and devices are we considering:
Modern memory elements for computers.
Efficient energy conversion by fuel cells.
2. Our research.
3. Collaboration with other labs.
4. Two openings for MSc with possible start as a project.
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1. Introduction, what kind of problem and devices are we considering.
1.1 Memory elements in computers:
a. Originally those elements were small magnets, 0 meant magnetization in one direction, 1 meant magnetization in the opposite direction.
b. Common elements today use transistors of the MOS (metal ‐ oxide =insulator ‐
semiconductor) type with the controlling signal being changed by charging an electrodes buried in the insulator.
c. The industry is looking for faster, less energy consuming and physically smaller
elements. d. One solution is to switch the state of a nano size solids between crystalline and amorphous state, provided the two states have significantly different resistance.
e. A more recent and more promising option is to change the resistance of a nano size solid by changing its composition. We concentrate on those. 2
g. Changing the resistance of a nano size solid by change of composition‐ memristive
devices (memristive = memory resistive):
Ag+
Ag dendrite is growing from the ion blocking electrode, gold, towards the silver electrode in As2S3. (Y. Hirose, H. Hirose, J. Appl. Phys. 47 (1976) 2767).
Once a full filament is formed short circuiting the electrodes, the resistance drop abruptly.
Sizes: of the order of a few mm.
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Example of the I‐V relations with switching for an element consisting of the simple structure: metal1|oxide | metal2
Pt|TaOx|Ta
Pt|TaOx|Ta
J.J. Yang et al., APL, 97 (2010) 232102
When the current is stopped the high or low resistance is maintained for a long time.
It can be probed using a low voltage (V<< 1V).
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h. This kind of experimental results raises a few question:
1) The oxide is an insulator (Egap~4eV) how can it exhibit a current
at room temperature?
2) How can it change composition?
3) What does “x” (x<<1) in TaOx mean? Is the composition not dictated by simple laws of chemistry:
that the ratio between the oxygen and metal in a compound has to be a simple number like Ta2O5 (≡ TaO2.5)?
4) Last and not least how does a memory effect arise?
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i.
Answers:
1) Let us start with question (3), what does “x” (x<<1) mean: • The composition of a compound need not follow the rule mentioned before. Oxygen can leave an oxide. • For example is CeO2 large changes in oxygen concentration up to 10% are possible, CeO1.8at T> 450oC without a phase change.
• How is charge balanced if the ionic charges are Ce4+ and O2‐ ? The missing oxygen ion is replaced by two electrons!
The ion O2‐ leaves the oxide as a neutral atom joining another oxygen atom to form the molecule O2 in the gas phase. The two electrons of the ion are left behind in the oxide.
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• Oxygen vacancies as donors.
Two oxygen ions leave the oxide as a neutral molecule O2 leaving behind 4 electrons, 2O2‐
→O2 +4e‐.
The electrons are left on the cations surrounding of the oxygen vacancies. This is denoted as “electrons in oxygen vacancies”. The vacancy is an electronic traps.
The trapped electrons may be excited at elevated temperature to the conduction band (and propagate as small polarons)
C.B. – Conduction band
C.B.
V.B. – valence band
VOx
VO∏
VO∏∏
V.B.
VOx, VO∏, VO∏∏ – oxygen vacancy, not ionized, singly ionized and doubly ionized, respectively
‐ electron
We use the Kröger‐Vink notation of defects. The charge is measured relative to the perfect, unperturbed lattice. x – relative neutral, ∏ ‐ one positive relative charge.
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• Electrons can be transferred to the conduction band also by tunneling under a strong electrical potential gradient (Poole – Frenkel emmision).
Thermal excitation: No. 5
Tunneling from trap to C.B.: No. 6
Trap to trap tunneling (Mott transition): No. 7
S. Yu et al. APL 99 (2011) 063507
• When their density is high they can undergo a Mott (insulator‐metal) transition.
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• Why does the nano size play a role?
The voltage applied is less than 10 Volts.
The (average) gradient on a 1cm solid is: E = = 1 Volt/cm
V
on a 10nm solid it is: E = 107 Volt/cm
Hence:
I)
The probability for tunneling from a trap to the conduction band is much higher.
II) The drift velocity in the conduction band v∂E is significantly higher.
III) The distance the electron has to propagate is significantly shorter.
Conclusion and answer to another question:
Conduction of electrons (and holes) in the nano size insulator (oxide) is possible at room temperature. 9
2) How can an oxide conduct oxygen ions?*
1. An ion needs a place to jump to. A nearby vacancy is excellent.
O vacancy
By loss of oxygen (reduction):
i.e. change of stoichiometry, ½O2
Ce:O ratio in example: CeO2‐x
+
Introduction of oxygen vacancy by doping and self compensation:
Ce4+
O2- ion
2 electrons
O vacancy
Ca2+
Zr4
+
O2- ion
Example: CaO doped ZrO2
1.Needs energy to overcome a potential barrier by temperature, A vacant place
for an O2- ion
to hop to
O2-
or by lowering the potential barrier applying a high driving force (easily achieved in the nano solid). * I. Riess, in: CRC Handbook of Solid State Electrochemistry, 1997
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• Conclusion and answer to another question:
Propagation of ions (oxygen ions) within the solid is possible, even at room temperature.
This allows also change in composition by exchange of oxygen with the neiborhood
at the surface.
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3) What about memory?
Or, how is it possible that the resistance change persists for many years?
There are two mechanisms that allow this:
I)
Heating by the current applied which enhances the mobility of the ions and the composition changes. After the current is switched off the solid cools back to room temperature and composition changes become very slow. Reading the resistance is done under a low current so that heating can be neglected. II) Having (at constant temperature) a mechanism which is very non linear in the applied voltage. It allows significant composition changes under a high voltage and negligible changes under a low voltage. This topic is still under investigation and interests us.
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• Example of heating effect in Cu|Cu2O|Ag, starting at 300K, under high V:*
1Ω
Current limited to ~1A
Thermal
80 Ω
Switching from high resistance. 80Ω, to low resistance 1Ω. This is attributed to ion, Cu+, motion, changes in the space charge and stoichiometry, accelerated by heating.
* Z. Rosenstock & I. Riess, Solid State Ionics, 136–137 (2000) 921.
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1.2 Efficient energy conversion by fuel cells.
Seemingly a very different topic is energy conversion. However, we shall show the close connection between this topic and that of memristive
devices.
Energy conversion: the generation of electrical energy from chemical energy contained in the oxidation reaction of fuel, e.g.
2H2 + O2 → 2H2O
If we burn hydrogen (H2) only heat is generated which can be converted to mechanical or electrical energy but with limited efficiency due to Carnot’s theorem. In reality the efficiency of an internal combustion engine is much less than this upper limit (~25% for cars, ~ 45% for large diesel engines).
A direct conversion of chemical energy to electrical at constant T could be much more efficient.
It needs to be an electro‐chemical reaction ! This is achieved in fuel cells.
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Example of applications of fuel cells (FC):
• Fuel cells are considered for use on a wide range, for: mobile phones, cars, submarines, etc.
• To drive cars: replace the internal combustion engine by a FC plus electrical motors Electrical motor
Fuel tank
Fuel cell
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• The world’s first submarine to be equipped with this unique propulsion system. It has already impressively proved its operating efficiency in extensive trials in a Germany Navy submarine.
15 PEM‐FC submarines have been ordered by four countries.
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• Principle of operation of a fuel cell (FC):
Fuel
Air
SE
Cathode
Anode
-
+
SE – solid electrolyte = ion conductor with negligible electron conduction.
Motor
Example: H2 as fuel and an oxygen ion conductor:
Fuel (H2)
Air
H2+O2H2O + 2e-
-
O2+4e2O2-
O2+
eMotor
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(3) (2) (1)
Fuel (H2)
Air
+O2-
H2
H2O + 2e-
O2+4e2O2-
O2
-
-
+
e-
Motor
Three processes are of interest:
(1) The reduction of oxygen at the cathode: O2+4e- → 2O2- and the incoporation of
the ion O2- into the solid membrane (solid electrolyte - SE).
(2) The conduction of oxygen ions in the SE.
(3) The oxidation reaction of the fuel by oxygen ions emerging from the SE, at the
anode.
Question (2) was already answered. Use a doped oxide in which vacancies are
generated without the generation of conducting electrons.
O
2
-
Question (1) and even more so (3) are complex. We shall demonstrate problem (1):
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In the porous electrode the gas has to diffuse within the pores to reach the SE. 19
• It is difficult and sometimes impossible to determine the steps actually taking place.
• One normally can determine the nature of the rate determining step.
• Then comes the question where? On which part of the electrode does this step take place?
• What is the role of oxygen vacancies at the surface?
• Can the reaction be made selective?
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The best electrodes are those which conduct both oxygen ions and electrons (or holes).
These are denoted as Mixed‐Ionic‐Electronic‐Conductors – MIECs.
They are the link between the energy conversion research and the research on memristive
devices.
SE (solid electrolytes) are a special case of MIEC in which the electronic conductivity is negligible.
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2. Our research or what do we research in the two fields presented before?
2.1 Memristive devices:
We are engaged in both theoretical and experimental research.
Theoretical:
• We analyze the characteristics of devices of the form: Metal1|MIEC|Metal2
devices under constant temperature.
• We calculate the current‐voltage relations and defect distributions inside the MIEC.
• The answer depends on the nature of the MIEC and the nature of the electrodes, if blocking for material exchange or not.
Experimental:
We measure the I‐V relations for systems we investigate theoretically.
Both dc and ac I‐V relations are measured.
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Examples of calculations and measurements done in our lab
1Ω
• One example was presented before which included heating: 80 Ω
• I‐V relation under periodic potential
Exp. Cyclic IV relations (T=47oC) Au|MoO3‐x)Au
Theoretical ac I‐V relations1
50.00
40.00
30.00
4th cycle
8th cycle
20.00
10.00
I [mA] 0.00
‐10.00
‐2.5 ‐2.0 ‐1.5 ‐1.0 ‐0.5
0.0
0.5
1.0
1.5
2.0
2.5
‐20.00
‐30.00
‐40.00
Scan rate: 0.2V/s
‐50.00
Voltage [V]
Example of the solution of the model for the
metal|MIEC|metal device with blocking electrodes,
one type of mobile ionic defect and one electronic
defect. The model predicts a negative resistance.
The axes are shown in normalized units.
1 Leshem A., Gonen
E., Riess I. 2011, Nanotechnology, 22, 254024.
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• Thin film arrangement Au|MoO3‐x|Au
Upper Au
electrodes
Middle layer of
MoO3-x
Lower Au
electrodes
• Change in MoO3‐x composition under applied current current:
SEM secondary electrons Au
Au
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• In memristive devices the I‐V curve has to exhibit line crossing.
We examined conditions leading to this crossing.1
1. D. Kalaev
and I. Riess, Solid State Ionics, 241 (2013) 17, ibid, in press, available on line.
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2.2 Research activity on the topic of energy conversion
• Both experimental and theoretical research takes place.
• Current research is concentrated on electrode processes.
Electrode problems are the more challenging ones but the reward for finding answers is higher.
Theoretical topics:
• What is the role of oxygen vacancies in the surface reaction?
• Can the electrodes be made selective to either the one or the other electrode reaction. Because if so the fuel cell design could be simplified, leading to drastic improvements in the energy density (per volume), fuel utilization and reduced manufacturing costs. Experimental topics:
• The effective activity range near a metallic electrode (triple phase boundary width).
•
18O
2 isotope exchange at the electrodes for determining some of the electrode processes. 27
3. Collaboration with other labs:
a. With Prof. Dan Ritter, Electrical Engineering, Technion, and colleagues in Germany on memristive devices.
b. With Prof. Avner Rothschild, Material Science, Technion, on memristive devices.
c. With Prof. Yoed Tsur, Chemical Engineering, Technion, on energy conversion (fuel cells)
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4. Two openings for MSc with a possible start as an undergraduate project on memrsitive
devices.
a. In may lab there is an opening for a graduate student to join PhD student Dima Kalaev
and complement his research. •
The samples are thin films.
•
The topic are experimental or theoretical (device characteristic). •
One can combine theoretical with experimental research in a good thesis.
•
One can start the research as a undergraduate project with obviously limited scope which will be either theoretical or experimental. b. In collaboration with Prof. Dan Ritter EE, in his lab (but MSc in physics) on memristive
devices:
Role of local temperature in the switching kinetics of resistive memory materials • A special solid state arrangement was devised that measures locally (on the nano scale) the temperature from the rate at which electrons are excited into the conduction band and contribute to the current there. • It is possible to do first an undergraduate project (with OK of our Physics Department).
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• In the summer another opening is expected on the topic of electrode processes using 18O
2 isotope exchange in collaboration with Prof. Yoed
Tsur in his lab (but MSc in physics).
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THANK YOU
Haifa by night
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