Organic

Subscriber access provided by Mount Allison University | Libraries and Archives

Organic Electronic Devices

Strain effects on the energy level alignment at metal/organic semiconductor interfaces Ainhoa Atxabal, Stephen McMillan, Beñat Garcia Arruabarrena, Subir Parui, Roger Llopis, Fèlix Casanova, Michael E. Flatté, and Luis E Hueso ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21531 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Strain effects on the energy level alignment at metal/organic semiconductor interfaces Ainhoa Atxabal1, Stephen R. McMillan2, Beñat García-Arruabarrena1, Subir Parui1,+, Roger Llopis1, Felix Casanova1,3, Michael E. Flatté2, Luis E. Hueso1,3* 1 CIC

nanoGUNE, 20018 Donostia-San Sebastian, Basque Country, Spain.

2

University of Iowa, Department of Physics and Astronomy, 203 Van Allen Hall, 52242-1479 Iowa City, Iowa, United States. 3 IKERBASQUE,

Basque Foundation for Science, 48013 Bilbao, Basque Country, Spain.

* [email protected] + Present address: IMEC and K. U. Leuven, Leuven, Belgium Key words Spectroscopy, in-device, metal/organic interface, organic semiconductors, flexible, energy barriers, energy levels Abstract Flexible and wearable are among the upcoming trends in the opto-electronics market. Nevertheless, bendable devices should ensure the same efficiency and stability as their rigid analogs. It is well known that energy barriers between the metal Fermi energy and the molecular levels of organic semiconductor devoted to charge transport are key parameters in the performance of organic based electronic devices. Therefore, it is paramount to understand how the energy barriers at metal/organic semiconductor interfaces change with bending. In this work we experimentally measure the interface energy barriers between a metallic contact and small semiconducting molecules. The measurements are performed in operative conditions while the samples are bent by a controlled applied mechanical strain. We determine that energy barriers are not sensitive to bending the sample, but we observe that the hopping transport of the charges in flat molecules can be tuned by mechanical strain. The theoretical model developed for this work confirms our experimental observations.

Introduction Flexible wearable devices based on organic semiconductors are emerging products in the current opto-electronics market 1-6. Large area, extreme thinness, and compliance to curved surfaces are 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the key requirements targeted for both functional passive and active electronic devices3,7–9. Consequently, it is paramount to understand the relation of the electronic properties and the performance of organic based devices with the mechanical strain. For instance, energy barriers build up between the metal Fermi energy, EF, and the molecular levels devoted to charge transport. These barriers limit the charge injection into the organic layers and they have a deep impact on the performances of the devices10–13. Therefore, a proper characterization and determination of their energetics is necessary for the design and optimization of metal–molecule interfaces. Recently, Wu et al. reported that mechanical strain modifies the work function of organic semiconductors 14. In such work, the authors induced mechanical strain by playing with the thermal expansion mismatch between a poly(dimethylsiloxane) (PDMS) substrate and a rubrene crystal, while monitoring the work function changes by scanning Kelvin probe microscopy 14. Despite the deep implications of these results in the understanding of the connection between structural and electronic disorder in soft organic materials, the impact that the strain induces on the energy level alignment at metal/molecular interfaces is still under debate. A device approach could be very beneficial for advancing the field of organic electronics from a state of fundamental curiosity to technological application. In this communication we base on in-device hot-electron spectroscopy to experimentally measure the interfacial energy barrier at metal-electron transporting small molecules. The measurements are performed in operative conditions while the samples are bent by a controlled applied mechanical strain. In-device measurements provides a further understanding of the impact of the mechanical strain at the metal/organic semiconductor interfaces and permits us testing two main points for the organic electronics community. First, we probe the energy barriers at metal/organic semiconductor interfaces and show that they do not vary with the strain induced by bending. Second, we observe that even if the interface energy barriers at metal/semiconductor interfaces are strain independent, we can tune the charge hopping rate through the semiconductor by bending the device, which results in an increase of the measured collector current. As proof of principle, we focus on N,N′-Dioctyl-3,4,9,10perylenedicarboximide (PTCDI-C8), a planar molecule, and we support it with three-terminal devices based on C60, spherical molecule. The data is complemented with the development of a theoretical model, which reinforces our experimental observations. Our results provide a step further in the understanding of the effect of the mechanical strain in devices with direct impact on the organic electronics community and the engineering of novel flexible electronics.

2 ACS Paragon Plus Environment

Page 2 of 13

Page 3 of 13

Results and discussion

a

c

b E

B

IE

C

E

B

IC ≠ 0

VEB   Al

LU Eg MO

VEB   Al

Al

Al2O3

LUM O Eg HOM O Au PTCDI-C8

C

IE

IC = 0 Al2O3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HOM O Au PTCDI-C8 Al

VCB  

d

R

VCB  

e

d O

C8H17-N O

O

N-C8H17

C60

O

Figure 1. Device scheme. Electrons can tunnel from the emitter electrode (E) to the base metal (B) when a bias between these two electrodes, VEB, is applied. (a) if VEB is lower than the metal/organic semiconductor interface barrier, ∆, ballistic electrons cannot enter into the molecular orbitals devoted to charge transport, in this case the lowest unoccupied molecular orbital LUMO, and they will be reflected back to the base. No collector (C) current IC will be measured. (b) if the applied VEB is higher than , some of the hot electrons flows into the LUMO of the organic semiconductor and they will diffuse to the top metal contact. Non-zero IC will be measured. The collector and the base are kept grounded. (c) Photograph of a Kapton substrate with six devices bent on a sample holder with a bending radius, R. (d) Chemical structure of N,N′-Dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8). (e) Chemical structure of C60 fullerene. In-device hot-electron spectroscopy is based on a three terminal vertical solid-state device 15-19. This technique is the solid state variant of ballistic electron emission spectroscopy 20-26.The working principle is shown in Figure 1. In more detail, our three-terminal device is composed of an emitter, a base and a collector. The device structure has been grown on flexible Kapton tape in ultra-high vacuum (UHV) and 0.5 nm of Co2O3 has served as adhesion layer (see Experimental section for further device fabrication details) 26,27. 20 nm-thick aluminum contact is



used as emitter. This electrode is then plasma-oxidized in-situ to create an Al2O3 tunnel barrier. The base electrode consist of 15 nm evaporated gold. Gold was chosen for being a commonly used material for device contacts. Its air stability and noble properties make it a suitable metal for pre-patterned devices. A 100 nm-thick organic semiconductor serves as collector. We chose the well-established n-type semiconductors PTCDI-C8 and C60 as proof of principle. See their chemical structure in Figure 1d and e, respectively. A current IE is injected from the emitter to the device when a negative bias VEB is applied. The electrons after tunneling through the Al2O3 barrier are “hot” in the base because their energy is above the Fermi energy of the metal. A fraction of these hot electrons crosses the base ballistically without any significant energy attenuation. If VEB is lower than the Au/molecule interface barrier  (Figure 1a), the ballistic electron current is reflected at interface and will flow into the base terminal (IB). No current is measured at the collector (IC =0). On the contrary, if VEB is higher than the barrier  (Figure 1b), some of the hot electrons that arrive at the Au/molecule interface enter in the LUMO level of the n-type semiconductor and diffuse towards the top Al electrode. A current is measured in the collector (IC ≠0). Hot electron injection is only possible with negative VEB because in this device configuration the base electrode is kept at ground potential (Figure 1a, b). Importantly, the current IC is measured without any external applied bias between the base and collector, VCB, and thus, IC can be considered as a purely diffusive current. This is possible due to both the momentum of the injected electrons perpendicular to the Au/semiconductor interface and to the built-in potential created by sandwiching the molecular material with two metallic contacts with different work functions 15-19. In this device configuration, the energy level alignment between the Fermi level of the emitter and the base is controlled with the VEB (see Figure 1), while the energy alignment at the base/collector interface is naturally given by the metal/molecule interface energy barrier . The experiment has been repeated with the sample flat and bent for five different bending radii R = ∞, 40, 25, 15, 12.5 and 5 mm. The cylindrical sample holders shown in Figure 1c permits performing the experiments in-situ while the samples are bent and thus, while mechanical strain is applied. The induced mechanical strain on the samples increases with the smaller bending radii (see Supporting Note 1 and Table S1).


Page 4 of 13

Page 5 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) Emitter current IE measured in the Al/Al2O3/Au junction of the device by sweeping the emitter-base bias, VEB. The junction has been measured under different strain conditions by bending the sample onto semi cylindrical holders with radius R = ∞, 40, 25, 15, 12.5, 5 mm. (b) Average strain, Savg dependence of IE at VEB = 1 V. Figure 2a and b show the characteristic IE - VEB curve of the tunnel junctions of a three-terminal hot-electron molecular transistor fabricated on Kapton substrate at 300 K 26, 28 and bent for several radii. No significant change is observed in the IE between flat and bent conditions (see Figure 2a). The current slightly decreases with the bending radius getting a maximum reduction in IE of 33% between no-strain (flat) and maximum-strain (R = 5mm) cases. These differences are better seen in Figure 2b, where the IE current at VEB = -1 V is plotted for the strain induced by bending the sample. A plausible explanation of such an observation is that when thin films are bent, the separation between its metallic grains is enlarged compared to the case when the sample is flat and thus, the charge transport is hindered. This deformation appears to be inelastic since the dozens of devices studied in different chips do not recover their initial performance after bending them from R= 40 mm to R=5 mm.



Figure 3. Spectroscopy measurements at 300 K for bending radii R = ∞, 40, 25, 15, 12.5, 5 mm. (a) Hot electron current, IC, of the PTCDI-C8 based device vs. emitter-base bias, VEB, for different bending radius. (b) Dependence of IC (at VEB = 1.5 V) with the average strain, Savg, (c) Normalized IC / IC max - VEB curves of PTCDI-C8 based device for different bending radius. d) Normalized IC / IC max - VEB curves of C60 based hot-electron transistor for different bending radius Figure 3a shows the IC - VEB curve of PTCDI-C8 based three-terminal hot-electron devices measured when the sample was flat without any applied mechanical strain and under strain by varying the bending radius of the sample holder from 5 mm to 40 mm. Figure 3c represents IC normalized with the maximum collector current IC max, which shows more clearly the curves visibly overlap and do not show any difference in their shape and their onset, which indicates that the energy barrier at metal/organic semiconductor interface does not vary with strain. This result is strengthened by the results in Figure 3d where IC/IC max measurements of C60 based device are plotted for different bending radii. Interpolating the fit of the linear growth of IC to IC 6 ACS Paragon Plus Environment

Page 6 of 13

Page 7 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


= 0, the interface energy barrier of Au/PTCDI-C8 is determined to be ∆ = 1.1 ± 0.1 V, and ∆ = 0.95 ± 0.1 V in the case of Au/C60 interface. In both cases the device-to-device variation in each chip is lower than the measurement precision, while the maximum variation from chip to chip is ±0.1 V. This straightforward method for the extraction of the interface energy barrier at metal/organic semiconductor interfaces has been proven before for other semiconductors 15, 17, 18. The three-terminal measurements performed on Au/PTCDI-C8 and Au/C60 based hot-electron transistor show that the bending of the devices and the consequent strain does not affect the transport energy barrier alignment, Δ, at the metal/organic semiconductor interfaces. These results differ from the conclusions of Wu et al. 14, where changes in the work function of Rubrene crystal with mechanical strain were reported. In this last case the bending strain introduced disorder into a perfectly oriented molecular layer. The disorder creates new molecular sites with different oxidation and reduction energies compared to the crystal, which changes the interface density of states and consequently the interface energy level alignment. In our work polycrystalline organic thin films with already disorder at the metal/organic interface are studied. Therefore, then strain does not significantly augment the disorder at the interface and the energy level alignment at metal/organic semiconductor interface is unaffected. Figure 3a shows that contrary to the trend of the IE, IC increases with the applied strain and reaches a value 430% higher when the device is bent with R = 5 mm compared to the case when it is flat. This evolution is better visible in Figure 3b, where the IC at VEB = -1.5 V is plotted with the mechanical strain apply on the device. Since the current injected from the tunnel junction IE (see Figure 2b) is almost constant, the changes in IC should be related to the organic semiconductor material.

Figure 4. (a) Diagram of a flexed organic with radius of curvature R (top) and unbent representation with linearly varying force (bottom). The global basis (un-primed) and local basis (primed) are shown with the angle 𝜃 defining the degree of rotation in the 𝑥 ― 𝑦 plane. 𝑥 is the axis of applied strain and 𝑤max is the distance between the neutral axis and the contact surface (100 nm) (b) The fractional change in resistivity as a function of the radius of curvature for experimental measurements (circle) and theoretical model (solid line). For specific values see Table S2.



Page 8 of 13

In order to disentangle the physics behind our results we have developed a theoretical model that considers the resistivity of the organic semiconductor with the bending of the device (see Figure 4a). For conjugated organics, the sp2 hybridized orbitals are split in energy from the unhybridized p-orbital. This energy splitting results in a HOMO and LUMO with p-type symmetry. In planar molecules like PTCDI-C8, the cores are composed of conjugated ring-like segments with well-defined spatial orientation along the axis of the un-hybridized orbitals. As the primary transport orbitals in these materials lack polar symmetry, the angular dependence of the wave function overlap between neighbouring sites is a key component in transport calculations In 𝜋-conjugated materials with well-defined site orientations and Miller-Abrahams type network the inter-site resistance can be considered as 𝑅0𝑖𝑗 𝑅𝑖𝑗(𝑟,𝜃) = exp(2𝑟𝑖𝑗/𝑎). (1) cos2 𝜃𝑖𝑗 where 𝑅0𝑖𝑗 describes a minimum resistance limited by material properties of the system, 𝑟𝑖𝑗 is the inter-site spacing, 𝜃𝑖𝑗 is the relative angle between the transport orbitals at sites i and j, and a is the carrier wave function localization radius. When the organic semiconductor is flexed, parameters 𝑟𝑖𝑗 and 𝜃𝑖𝑗 change, and the molecular unit cell is strained with the volume, in general, not conserved. This results in a modification to the volumetric density of states which enters the transport equations exponentially through the intersite spacing 𝑟𝑖𝑗. For a change in length dx along the applied strain axis the material response along the perpendicular axes (dy) are governed by the Poisson’s ratio, 𝑑𝑦 = ―𝜈 𝑑𝑥. The current through the organic layer exhibits a hopping conduction along a path of sites with each hop occurring at a rate that depends on the relative position and orientation of the initial and final site. These rates are analogous to resistors in series with a value corresponding to Eq. (1). Since the bulk resistance is determined by summing the individual resistances along the path it is clear from Eq. (1) that hops between sites with relative orientations near perpendicular will dominate the total value. These bottlenecks that exist in the relaxed organic will be broken as the device is bent and the sites shift their relative position and orientation. Since the strain is dependent on the distance from the neutral axis the average value is considered, 𝑆avg =

𝑤max 2𝑅

,

where we have assumed the neutral axis to be midway between the top and bottom contacts and therefore 𝑤max = 100 nm. For two neighbouring sites i and j in a bottleneck configuration the orientation of site i and site j 𝜋

with respect to the global basis are related through 𝜃𝑗 = 𝜃𝑖 + 2 ―𝜉, where 𝜉 is a small deviation from completely perpendicular orientation. The inter-site resistance for a bottleneck in a relaxed organic can then be written as


Page 9 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exp 𝑅∞⊥

=

𝑅0𝑖𝑗

[

(

2𝑟𝑖𝑗

( ) 𝑎

)

)]

(

𝜋 𝜋 sin 𝜃𝑖 sin 𝜃𝑖 + ― 𝜉 + cos 𝜃𝑖cos 𝜃𝑖 + ― 𝜉 2 2

.

(2)

2

Therefore, the resistivity of the flat organic is exp 𝜌∞⊥

∝

[

(

)

2𝑟𝑖𝑗

( ) 𝑎

)]

(

𝜋 𝜋 sin 𝜃𝑖 sin 𝜃𝑖 + ― 𝜉 + cos 𝜃𝑖cos 𝜃𝑖 + ― 𝜉 2 2

2

(3)

and the resistivity of the deformed organic is 1 2𝑟𝑖𝑗 (1 + (1 ― 2𝜈)𝑆avg )3 exp 𝑎 𝜌𝑅⊥ ∝ 2 . (4) sin [𝜉 ― 𝑆avg (𝜈 + 1)cos (𝜉)sin (2𝜃𝑖 ― 𝜉)]

[

]

In comparing with experimental values, the fractional change in resistivity is considered ∞ 𝑅 𝛥𝜌 |𝜌 ⊥ ― 𝜌 ⊥ | = (5) 𝜌∞⊥ 𝜌∞⊥ as a function of the radius of curvature. The value of the inter-site spacing is approximated from the cubed root of the volume of the measured triclinic unit cell for PTCDI-C8 as 9.19 Å 29 and the carrier localization radius is approximated as 1 Å. The value for the Poisson’s ratio is estimated from measured values of similar materials to be 0.4 and the angular deviation 𝜉 is left 𝜋

as a fitting parameter. For bottlenecks in the relaxed organic with relative angle 𝜃 ⊥ = 2 ―𝜉 the general experimental trend can be reproduced for 𝜉 = 2.5 × 10 ―6 rad as seen in Fig(4b), where the experimental and theoretical

𝛥𝜌 𝜌∞⊥

are plotted with respect to the R. See Supporting Note 2 for

further details. These results are valid even for cases involving anisotropic transport since in the bottleneck configuration rotational dependence of the sites dominates the resistivity. Thus even for systems with anisotropic spacings between sites and anisotropic site wave function decay lengths we obtain nearly identical numerical results. See Supporting Note 3 and Figure S1 and S2 for details regarding these calculations. Even if this observation is new for organic semiconductors, changes in the electronic structure of organic superconductors and charge transfer salts with strain have been previously reported 30-32. Conclusions In conclusion, we have successfully built three-terminal hot-electron molecular transistors on flexible Kapton substrates based on Au/PTCDI-C8 and Au/C60, and measured them flat (no



strain) and flexed (under strain). Subsequently, we have demonstrated that the energy barrier between the EF of the metallic contact and the LUMO level, Δ, is strain independent, i.e. ∆ = 1.1 ± 0.1 eV for Au/PTCDI-C8 and ∆ = 0.95 ± 0.1 eV for Au/C60 interfaces. Besides, we have demonstrated that the bending of the planar organic semiconductor reorients the molecules and breaks the bottlenecks, which is reflected experimentally as an increase in the amplitude of the measured IC. These results give a further understating of the interfacial energy parameters for the engineering of flexible organic electronic devices as well as present a simple method for manipulating the detected current in organic based devices.

References (1) (2)

(3)

(4) (5)

(6)

(7) (8)

(9)

(10)

(11)

Forrest, S. R. Electronic Appliances on Plastic. Nature 2004, 428, 911–918. Wang, Y.; Zhu, C.; Pfattner, R.; Yan, H.; Jin, L.; Chen, S.; Molina-Lopez, F.; Lissel, F.; Liu, J.; Rabiah, N. I.; et al. A Highly Stretchable, Transparent, and Conductive Polymer. Sci. Adv. 2017, 3, 1602076–1602086. Karnaushenko, D.; Makarov, D.; Stöber, M.; Karnaushenko, D. D.; Baunack, S.; Schmidt, O. G. High-Performance Magnetic Sensorics for Printable and Flexible Electronics. Adv. Mater. 2015, 27, 880–885. Rogers, J. A.; Someya, T.; Huang, Y. Materials and Mechanics for Stretchable Electronics. Science 2010, 327, 1603–1607. Xu, J.; Wang, S.; Wang, G. N.; Zhu, C.; Luo, S.; Jin, L.; Gu, X.; Chen, S.; Feig, V. R.; To, J. W. F.; et al. Highly Stretchable Polymer Semiconductor Films Through the Nanoconfinement Effect. Science 2017, 355, 59–64. Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.; Graz, I.; Bauer-gogonea, S.; Bauer, S.; Someya, T.; et al. An Ultra-Lightweight Design for Imperceptible Plastic Electronics. Nature 2013, 499, 458-463. Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Flexible Organic Transistors and Circuits with Extreme Bending Stability. Nat. Mater. 2010, 9, 1015–1022. Hwang, S. W.; Tao, H.; Kim, D. H.; Cheng, H.; Song, J. K.; Rill, E.; Brenckle, M. A.; Panilaitis, B.; Won, S. M.; Kim, Y. S.; et al. A Physically Transient Form of Silicon Electronics. Science 2012, 337, 1640–1645. Salvatore, G. A.; Mu, N.; Kinkeldei, T.; Petti, L.; Zysset, C.; Tro, G.; Strebel, I.; Bu, L. Wafer-Scale Design of Lightweight and Transparent Electronics That Wraps around Hairs. Nat. Commun. 2014, 5, 2982–2990. Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Brédas, J. L.; Lögdlund, M.; et al. Electroluminescence in Conjugated Polymers and Fullerenes. Nature 1999, 397, 121–128. Sirringhaus, H. Device Physics of Solution-Processed Organic Field-Effect Transistors. 10 ACS Paragon Plus Environment

Page 10 of 13

Page 11 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) (13) (14)

(15)

(16) (17)

(18)

(19)

(20) (21) (22) (23) (24)

(25)

Adv. Mater. 2005, 17, 2411–2425. Hwang, J.; Wan, A.; Kahn, A. Energetics of Metal – Organic Interfaces : New Experiments and Assessment of the Field. Mater. Sci. Eng. R 2009, 64, 1–31. Braun, B. S.; Salaneck, W. R.; Fahlman, M. Energy-Level Alignment at Organic / Metal and Organic / Organic Interfaces. Adv. Mater. 2009, 21, 1450–1472. Wu, Y.; Chew, A. R.; Rojas, G. A.; Sini, G.; Haugstad, G.; Belianinov, A.; Kalinin, S. V.; Li, H.; Risko, C.; Brédas, J. L.; et al. Strain Effects on the Work Function of an Organic Semiconductor. Nat. Commun. 2016, 7, 10270–10278. Atxabal, A.; Braun, S.; Arnold, T.; Sun, X.; Parui, S.; Liu, X.; Gozalvez, C.; Llopis, R.; Mateo-alonso, A.; Casanova, F.; et al. Energy Level Alignment at Metal / SolutionProcessed Organic Semiconductor Interfaces. Adv. Mater. 2017, 10, 1606901–1606906. Arnold, T.; Atxabal, A.; Parui, S.; Hueso, L. E.; Ortmann, F. Hot Electrons and Hot Spins at Metal-Organic Interfaces. Adv. Funct. Mater. 2018, 20, 1706105–1706115. Gobbi, M.; Pietrobon, L.; Atxabal, A.; Bedoya-Pinto, A.; Sun, X.; Golmar, F.; Llopis, R.; Casanova, F.; Hueso, L. E. Determination of Energy Level Alignment at Metal/Molecule Interfaces by In-Device Electrical Spectroscopy. Nat. Commun. 2014, 5 (May), 4161– 4168. Parui, S.; Atxabal, A.; Ribeiro, M.; Bedoya-Pinto, A.; Sun, X.; Llopis, R.; Casanova, F.; Hueso, L. E. Reliable Determination of the Cu/n-Si Schottky Barrier Height by Using InDevice Hot- Electron Spectroscopy. Appl. Phys. Lett. 2015, 107 183502–183505. Jiang, J. S.; Pearson, J. E.; Bader, S. D. Direct Determination of Energy Level Alignment and Charge Transport at Metal - Alq3 Interfaces via Ballistic-Electron-Emission Spectroscopy. Phys. Rev. Lett. 2011, 106, 156807–156811. Kaiser, W. J.; Bell, L. D. Direct Investigation of Subsurface Interface Electronic Structure by Ballistic-Electron-Emission Microscopy. Phys. Rev. Lett. 1988, 60 (14), 1406–1410. Bell, L. D.; Kaiser, W. J. Observation of Interface Band Structure by Ballistic-ElectronEmission Microscopy. Phys. Rev. Lett. 1988, 61 (20), 2368–2371. Ludeke, R.; Prietsch, M.; Samsavar, A.; Ballistic Electron Emission Spectroscopy of Metals on GaP(110). J. Vac. Sci. Technol. B 1991, 9 (110) 885-890. Bell, L.; Ballistic Electron Emission Microscopy and Spectroscopy: Recent Results and Related Techniques. J. Vac. Sci. Technol. B 2016, 34 (4), 040801 -040828. Yi, W.; Narayanamurti, V.; Lu, H.; Scarpulla, M. A.; Gossard, A. C.; Huang, Y.; Ryou, J. H.; Dupuis, R. D.; Bandgap and Band Offsets Determination of Semiconductor Heterostructures Using Three-Terminal Ballistic Carrier Spectroscopy. Appl. Phys. Lett. 2009, 95 (11), 112102-112105. Yi, W.; Narayanamurti, V.; Lu, H.; Scarpulla, M. A.; Gossard, A. C.; Probing Semiconductor Band Structures and Heterojunction Interface Properties with Ballistic Carrier Emission: GaAs/ Alx Ga 1-x As as a Model System, Phys. Rev. B 2010, 81, 235325-235338. 11 ACS Paragon Plus Environment


(26)

(27) (28)

(29)

(30)

(31)

(32)

Sun, X.; Bedoya-Pinto, A.; Llopis, R.; Casanova, F.; Hueso, L. E. Flexible SemiTransparent Organic Spin Valve Based on Bathocuproine. Appl. Phys. Lett. 2014, 105, 083302-083306. Bedoya-Pinto, A.; Donolato, M.; Gobbi, M.; Hueso, L. E.; Vavassori, P. Flexible Spintronic Devices on Kapton. Appl. Phys. Lett. 2014, 104, 062412-062417. Sun, X.; Gobbi, M.; Bedoya-pinto, A.; Txoperena, O.; Golmar, F.; Llopis, R.; Chuvilin, A.; Casanova, F.; Hueso, L. E. Room Temperature Air-Stable Spin Transport in Bathocuproine-Based Spin Valves. Nat. Commun. 2013, 4, 2794–2797. Briseno, A. L.; Mannsfeld, S. C. B.; Reese, C.; Hancock, J. M.; Xiong, Y.; Jenekhe, S. A.; Bao, Z.; Xia, Y. Perylenediimide Nanowires and Their Use in Fabricating Field-Effect Transistors and Complementary Inverters. Nano Lett. 2007, 7 (9), 2847–2853. Kawasugi, Y.; Yamamoto, H. M.; Hosoda, M.; Tajima N.; Fukunaga,T.; Tsukagoshi, K.; Kato, R.; Strain-Induced Superconductor/Insulator Transition and Field Effect in a Thin Single Crystal of Molecular Conductor. Appl. Phys. Lett. 2008, 92, 243508-243511. Maesato, M.; Kaga, Y.; Kondo, R.; Kagoshima, S.; Uniaxial strain method for soft crystals: Application to the Control of the Electronic Poperties of Organic Conductors. Rev. Sci. Instrum. 2000, 71, 176-181. Kondo, R.; Kagoshima, S.; Maesato, M.; Crystal Structure and Electronic Band Structure of the Organic Superconductor α−(BEDT−TTF)2NH4Hg(SCN)4 under Uniaxial Strain. Phys. Rev. B. 2003, 67, 134519-134525.

Experimental section Device fabrication: All the devices described in this work where fabricated in ultra-high vacuum (UHV) evaporator chamber (base pressure

Organic

Recommend Documents