Effect of Tunneling Electron Injection on the Dynamic Motion of

Article pubs.acs.org/JPCC

Effect of Tunneling Electron Injection on the Dynamic Motion of Confined Molecules in Self-Assembled Molecular Corrals Hirulak D. Siriwardena and Masaru Shimomura* Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Nakaku, 432-8011, Hamamatsu, Japan S Supporting Information *

ABSTRACT: The dynamic motion of trimethylphosphine (TMP) adsorbates enclosed in self-assembled pyrrole molecular corral structures was observed using scanning tunneling microscopy (STM) techniques. Sequential STM images were obtained under various sample bias and tunneling current conditions to investigate the influence of tunneling conditions on the TMP dynamic motion. It was observed that the TMP adsorbates attempt to move more frequently at lower sample bias conditions, and the TMP movement holds a linear-like relationship with the sample bias. The Si−P antibonding orbitals get stimulated at lower sample bias conditions, which in turn weaken the Si−P bond and force the TMP adsorbates to move. A process can be developed using this attribute to control the dynamic TMP motion such that this molecular system can be utilized as a molecular electronic device.

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INTRODUCTION The modern electronic industry is evolving each day, making the impossible a reality. One of the best examples is the advancement in quantum computing in the recent years. Dwave, the first commercial quantum computer, was first introduced in May 2011 followed by the introduction of an improved design in April 2014, just three years after its first release.1 Molecular electronics has received much attention due to its potential to be used as sophisticated qubit-based molecular computing devices. The concept of the modern single-molecular device was proposed by Aviram and Ratner which functions as a diode, utilizing the donor−acceptor structure of an organic molecule to rectify the electric current flow in one direction.2 Currently, many researchers have focused on developing experimental techniques to investigate the properties of the single molecules, in order to fabricate different functional devices. Scanning tunneling microscopy (STM) is one of the well-developed experimental methods which allows the fabrication of real molecular electronic devices applying theoretical ideas. Devices with various electronic functions can be tailored using single molecules based on their inherent properties and by using suitable chemical designs and appropriate synthetic methodologies.3 Researchers have been able to observe a vast range of characteristic functions of single © 2017 American Chemical Society

molecules, and various molecular electronic devices have already been developed. Molecular level diodes,4,5 transistors,6,7 switches,8,9 and electronic memory devices10,11 have been designed and developed, and the methodologies have been well documented. The current demand in the electronic industry for miniaturizing electronic devices can be addressed successfully by the progression of single molecular electronics. The ultimate goal is not only to produce single-molecular devices to be used in quantum computers or to replace typical silicon-based electronic components but also to find an opportunity to study and understand the materials and their reactions at the molecular level.12 The fabrication process is extremely challenging. The placement of molecules between two electrodes with angstrom-level precision is a tricky task. The molecular selfassembly property gets triggered due to the thermal motion, and the intermolecular attraction of the molecules can be used to fabricate complex single-molecular devices with ease over a large surface. The surface of Si(111)-(7 × 7) reconstruction is ideal as a natural template, and it can be used to form complex Received: November 17, 2016 Revised: February 14, 2017 Published: February 15, 2017 4980

DOI: 10.1021/acs.jpcc.6b11474 J. Phys. Chem. C 2017, 121, 4980−4988

Article

The Journal of Physical Chemistry C

possible driving forces yet to be determined. Tunneling current (It) conditions and sample bias (Vs) conditions can affect the electronic states of TMP molecules, manipulating its dynamic motion. In this study, the effects on the TMP dynamic motion were observed by changing the STM measurement conditions on a confined TMP adsorbate in the pyrrole molecular fence. This paper proposes that this TMP/pyrrole molecular arrangement has the potential application of a molecular switch if the motion of TMP adsorbates could be controlled by an external electrostatic field.

self-assembled molecular structures. Figure 1a illustrates the dimer-adatom-stacking-fault (DAS) model of Si(111)-(7 × 7)

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EXPERIMENTAL METHODS A previously reported experimental method was used in this study with some modifications.19 An n-type Si(111) wafer was cut into small pieces (1 mm × 7 mm) and was used as the substrate. The substrate surfaces were cleaned using HF etching (HF:deionized water = 1:20) and piranha solution (H2SO4:H2O2 = 4:1). STM (JEOL-4500XT) measurements were carried out using a chemically etched tungsten tip, and the base pressure of the main chamber was maintained at 1 × 10−8 Pa throughout the analyzing process. The Si(111) substrate was further cleaned using several cycles of flash heating at ∼1100 °C ensuring ultrahigh vacuum conditions, and the 7 × 7 reconstruction was confirmed using the STM images. Then, the substrate surface was directly exposed to purified pyrrole vapor, and while maintaining the dosage at 1.5 L (Langmuir, 1 L = 1 × 10−6 Torr·s). Afterward, the same substrate was exposed to 0.5 L of TMP gas. All of the STM measurements and imaging were carried out at room temperature without any modifications. The sequential STM images were acquired at Vs = +3.0, +2.5, +2.0, +1.5, and +1.0 V while keeping the tunneling current constant (=0.10 nA) to investigate the relationship between TMP movement and sample bias conditions. As the next step, sequential STM images were obtained under 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40 nA tunneling current conditions, respectively, under three different sample bias conditions (Vs = +2.5, +2.0, and +1.5 V) to determine the effect of the tip− sample distance on the TMP dynamic motion. Scanning tunneling spectroscopy (STS) measurements were also carried out alongside the STM analysis in order to study the molecular system in detail.

Figure 1. (a) Dimer-adatom-stacking-fault (DAS) model of the Si(111)-(7 × 7) unit cell, (b) filled state (Vs = −1.5 V) and (c) empty state (Vs = +1.5) STM images of a clean Si(111)-(7 × 7) surface.

proposed by Takayanagi et al. According to the DAS model, a unit cell of 7 × 7 reconstruction has 12 adatoms, 6 rest-atoms, and a corner hole providing 19 dangling bonds.13 The filled state and the empty state STM images of clean Si(111)-(7 × 7) reconstruction are shown in parts b and c of Figure 1, respectively. In the filled state image, corner-adatoms appear brighter than the center-adatoms. Furthermore, when considering half unit cells, faulted half adatoms appear brighter than unfaulted half adatoms. This availability of chemically, spatially, and electronically different sites which can be utilized for molecular adsorption is an advantage of using a 7 × 7 reconstruction.14 The Si(111)-(7 × 7) reconstruction is also stable and has a large unit cell which can be obtained by simply heating under ultrahigh vacuum conditions. Self-assembled molecular corrals have been prepared on a Si(111)-(7 × 7) surface using organic molecules such as haloalkanes and other various organic molecules.15,16 The main purpose of these studies is to analyze the possibility of fabricating a molecular-type switch which utilizes the molecular dynamics of haloalkanes.17 Zhang et al. prepared self-assembled pyrrole molecular corrals on a Si(111)-(7 × 7) surface and reported that pyrrolyl species tend to adsorb selectively on center-adatom positions.16 Our previous studies have shown that trimethylphosphine (TMP) molecules get attached selectively on the center-adatom positions as well. Also, the TMP molecules adsorbed on corner-adatom positions are dynamically moved on to center-adatom positions after a certain amount of time.18 This particular type of pyrrole molecular corrals can be used to prepare a single molecular electronic device by trapping a TMP molecule inside and utilizing its dynamic motion. By analyzing sequential STM images, it was observed that the confined TMP molecule tends to hop between vacant adatom sites within the corral.19 The dynamic motion is affected by the electrostatic repulsion between the TMP adsorbates, and other

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RESULTS AND DISCUSSION The STM images of the Si(111)-(7 × 7) surface acquired after exposure to pyrrole and TMP are shown in Figure 2. In order

Figure 2. (a) Empty-state STM image obtained after the pyrrole and TMP exposure (It = 0.1 nA). (b) Positions of TMP, pyrrolyl, and bare Si adatoms. 4981


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enhanced, and the brightness of pyrrole species gets decreased with the decrement of the sample bias voltage. After +1.5 V bias, it can be observed that the protrusions of the pyrrole species disappear from the STM images, and only the TMP adsorbates and Si adatoms can be seen, as shown in Figure 3e and f. It was also observed that the changes in the sample bias voltage have not significantly affected the brightness of the protrusions that arose due to the TMP adsorbates. A higher number of local density of empty electronic states present near the Fermi level of the TMP which accommodates tunneling electrons from the tip could be the reason for this unchanged brightness. On the other hand, pyrrole does not have enough empty states near the Fermi level to receive the incoming tunneling electrons, and therefore disappears from the STM images after +1.5 V bias. A TMP movement rate was defined as follows by counting the number of TMP adsorbates which moved to a different adatom position during the scanning period to distinguish and compare the effect of the STM measurement conditions on the dynamic motion of the TMP.

to understand the self-assembled molecular arrangement more clearly, Figure 2b is denoted with red, blue, and green markers corresponding to TMP adsorbates, pyrrolyl species, and bare Si adatoms, respectively. Protrusions due to TMP adsorbates appear the brightest owing to the high local density of empty states around it at positive sample bias conditions.19 It can be observed that the self-assembled pyrrole molecular corrals have formed uniformly on the substrate and also have the honeycomb structure as previously reported.16 Furthermore, the majority of the molecular corrals encloses one or more TMP adsorbates. However, some molecular corrals, as denoted by the yellow arrow in Figure 2b, did not have any TMP adsorbates. Even though the pyrrole adsorption was uniform, some vacant center-adatom positions were observed on the surface. Some of these vacant center-adatom sites were occupied by TMP adsorbates, as shown by the white arrow in Figure 2b. Figure 3 illustrates the STM images obtained over the same area by varying sample bias voltage. When the sample bias was

TMP movement rate number of TMP adsorbates moved (mov) = scanning duration (min)

The sequential STM images obtained under +3.0 V sample bias after exposing the substrate surface to pyrrole and TMP, respectively, are shown in Figure 4. A similar set of STM images acquired over the same scanning area was obtained for each sample bias condition, and is included in the Supporting Information. Enclosed TMP adsorbates are denoted with numbers in order to track their movement easily. Figure 4a shows the initial STM image, and the TMP molecules are numbered using the color blue. From Figure 4b and onward, the TMP adsorbates which moved to a different adatom position are marked using the color red. At this sample bias condition, a frequent TMP movement was not observed. Generally, most of the molecular corrals contain one TMP adsorbate. However, it was noted that some molecular corrals have more than one TMP adsorbate. The TMP adsorbates marked as 38−39 and 42−37, as shown in Figure 4, are two pairs of molecules enclosed in the same molecular corral. These adsorbates try to move more often due to the mutual electrostatic repulsive force that occurs along the direction parallel to the surface, and this agrees well with our previous study on this system.19 This tendency was also observed in other bias conditions as well (TMP adsorbates labeled as 38 and 39 in Figure S1 and Figure S2). However, the TMP adsorbates which get attached to vacant center-adatom positions rather than the corner-adatom positions as expected do not prefer to move regularly. This is attributed to the stronger bond formation between the center-adatom and the TMP if nearby empty rest-atom sites are available. Usually, when a pyrrole molecule gets attached to a centeradatom position dissociatively, the H atom in the pyrrole N−H bond binds to the nearest rest-atom position.20 When a complete pyrrole molecular corral is formed, the rest-atoms also get saturated by these H atoms, leaving only corner-adatom positions for the TMP molecules to form dative bonds. An unoccupied center-adatom position after pyrrole exposure indicates the availability of a vacant rest-atom position. This configuration is most favorable for TMP attachment, as the center-adatom site can charge transfer to the nearby empty rest-

Figure 3. STM images obtained under different sample bias voltages. It = 0.10 nA.

fixed at +3.0 V, the acquired STM images did not appear to be very clear even though it is possible to identify the molecules. Enclosed unreacted adatom positions cannot be seen at +3.0 V bias, but from +2.5 V bias onward, those unreacted adatom sites were visible in the STM images. Generally, the images get 4982


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Figure 4. Sequential STM images acquired under Vs = +3.0 V and It = 0.1 nA.

atom position and can fully accommodate the lone pair of the incoming TMP molecule to form a stronger bond. Therefore, TMP adsorbates at center-adatom positions like 37 and 43, as shown in Figure S1, rarely move to a different adatom position. The number of TMP adsorbates which are capable of moving increased with the decrement of the sample bias voltage. It was observed that some TMP adsorbates start to move even in between the pyrrole molecular corrals through defects. The

steric barriers imposed by the pyrrole molecular corral limit the movement of TMP adsorbates. If a defect exists in the molecular fence due to the absence of a pyrrolyl species on a center-adatom position, the TMP adsorbates can move in between these two adjacent corrals, since the steric barrier is ineffective. Such a situation is shown in Figure 5, and the TMP marked as 3 has switched between two adjacent corrals through a defect present in the fence. As shown in Figure 5k, at one 4983


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Figure 5. Movement of TMP adsorbents between two pyrrole molecular fences (Vs = +2.0 V, It = 0.10 nA). The pyrrolyl adsorbed center-adatom positions of corresponding molecular fences are denoted in blue color circles. For the complete sequential STM image set, please refer to Supporting Information Figure S2.

Figure 6. Selected set of sequential STM images from (a) Figure S3h−j obtained under Vs = +1.5 V and It = 0.10 nA and (b) Figure S4h−j obtained under Vs = +1.0 V and It = 0.10 nA. Please refer to Supporting Information Figure S3 and S4 for the complete set of sequential STM images obtained under these bias conditions.

Calculated movement frequencies of the TMP adsorbates at each sample bias condition are shown in Table 1. By analyzing

situation, three TMP adsorbates (numbered 3, 20, and 15) have occupied a single corral. This arrangement can impose an excess mutual repulsive electrostatic force. The TMP adsorbate numbered as 15 has moved over the steric barrier to the nearby corral, and this can be observed in Figure 5n. It is necessary to obtain defect-free molecular fence structures if possible to prevent such unexpected situations. Furthermore, it was observed that some TMP adsorbates disappear or move significant distances across the scanning area during the imaging process. Figure 6a illustrates a selected set of sequential STM images obtained under Vs = +1.5 V. Here, the TMP adsorbate marked as 44, shown in Figure 6a(h), has suddenly disappeared from the scanning area. The last known position of the adsorbate is shown in yellow color. Similarly, the TMP adsorbate marked as 45 in Figure 6b(h), obtained under Vs = +1.0 V, also has moved away from the scanning area. The effect of the tip on the TMPs cannot be completely neglected regarding these events. Even though there is no exponential change that occurs as in the case of the tunneling current, the tip−sample distance does reduce when the sample bias is decreased. After the adsorption, the TMP species is surrounded by a positive electrostatic potential due to the presence of methyl groups, and the negatively charged tip can pick loosely bonded TMP adsorbates and move it to another position. This could be the cause of the significant movement distances and sudden disappearances of some TMP adsorbates.

Table 1. Sample Bias Voltages and the TMP Movement Frequencies Vs (V)

TMP movement frequency (mov/min)

free-to-move TMP adsorbates (%)

3.0 2.5 2.0 1.5 1.0

9.20 24.32 32.43 42.67 60.00

20 36 43 63 75

the listed TMP movement frequency values, it can be determined that the TMP adsorbates try to move more frequently under lower bias conditions. Furthermore, it was observed that the number of free-to-move TMP adsorbates increases with the sample bias decrement. At +3.0 V sample bias, only about 20% of the enclosed TMP adsorbates have moved to a different adatom position. It can be suggested that, at this higher bias condition, only the thermal energy and the mutual repulsive forces drive the TMP adsorbates’ dynamic movement. However, an additional driving force also contributes to this at lower bias conditions. The antibonding orbitals of the Si−P bond may get involved, and this explains the frequent dynamic motion, and also the increment in the 4984


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The Journal of Physical Chemistry C number of free-to-move TMP adsorbates at the lower sample bias conditions. It has been pointed out that the Si−H bond shares similar characteristics with the Si−P bond and hence the bonding energies are similar.21 Therefore, when considering the Si−P bond, the bonding orbitals would entirely be occupied beyond the valence band minimum, but the antibonding orbitals would lie closer to the mobility edge of the conduction band.22 At lower bias conditions, the tunneling electrons will stimulate these antibonding orbitals, which lie closer to the conduction band, weakening the Si−P bonding and forcing the TMP adsorbates to move. It is possible to identify the TMP adsorbates even at lower sample bias conditions, since they have enough empty states to receive the incoming tunneling electrons. These empty states can also consist of antibonding orbitals. With this argument, it can be clarified why the number of free-to-move TMP adsorbates increases with the decrement of the sample bias voltage. Even though the Si−P bond is weakened, the TMP adsorbate does not have enough energy to get desorbed from the surface. A reduction of the number of TMP adsorbates in the scanning area could have been observed if the TMP desorption had taken place. However, a notable decrease in the number of TMP adsorbates was not observed, and the TMP would move on vacant adatom sites rather than getting desorbed. The data shown in Table 1 was plotted against the sample bias voltage and shown in Figure 7 for further analysis. An incremental nature can be observed in both curves when the sample bias is reduced.

Figure 8. First derivative of tunneling current against the sample bias of the TMP/pyrrole molecular system.

the involvement of the antibonding orbitals. The TMP−tip interactions which occur at low bias can lead to TMP decomposition which may explain the absence of peaks, as expected at conditions lower than +1.0 V bias. To further elucidate the response of the TMP movement, the tunneling current condition was varied while keeping the sample bias constant. According to the tunneling current equation, the tip−sample distance decreases exponentially when the tunneling current is increased.23 Figure 9 shows the STM images obtained under different tunneling current conditions over the same location at a fixed +2.0 V sample bias. There was no significant enhancement in the STM images when the tunneling current condition was increased, and the images are not comparable to STM images obtained when the sample bias was changed with fixed tunneling current. Hence, it can be concluded that the change in tip−sample distance has a lesser effect on the improvement of the STM image quality. Similarly to the previous occasion, sequential STM images were obtained at each tunneling current condition, and the TMP movement frequency was determined for each instance. However, to broaden the understanding of the involvement of the Si−P antibonding orbitals, the sequential STM imaging was carried out at three different sample bias conditions, i.e., Vs = +2.5 V (Figures S5−S11 in the Supporting Information), Vs = +2.0 V, and Vs = +1.5 V. The calculated STM movement frequencies are tabulated in Table 2 along with the percentage of free-to-move TMP adsorbates. It was challenging to obtain very clear STM images at biases lower than +1.5 V and tunneling current conditions higher than 0.35 nA, due to the high tip−sample interactions. For example, it was not possible to obtain clear sequential STM images at 1.5 V bias and 0.40 nA tunneling conditions. When the negatively charged tip is brought to a closer distance, positively charged TMP molecules tend to desorb from the surface and get attached to the tip, destroying the adsorption structure and limiting the dynamic motion of the TMP. As shown in Table 2, it was observed that there is no significant change in the TMP movement frequency under various tunneling current conditions and also in the percentage of free-to-move adsorbates. The TMP movement frequency and the free-to-move TMP adsorbate percentage are plotted against the scanned tunneling current condition at each bias in Figure 10. In each scenario, the TMP movement frequencies and free-to-move TMP adsorbate percentage curves follow the

Figure 7. Relationship of TMP movement frequency and free-to-move TMP adsorbates with sample bias voltage.

STS analysis of the molecular system was carried out to clarify the involvement of antibonding orbitals on the TMP movement frequency, and the first derivate of tunneling current plotted against the sample bias is shown in Figure 8. In the graph, a shoulder peak can be observed clearly between ∼1.5 and 2.0 V sample bias conditions. This shoulder peak corresponds to the density of states of the antibonding orbitals of the Si−P bond and also confirms availability of empty states to accommodate the incoming tunneling electrons. The molecular orbital energy of the Si−TMP bond calculated using density functional theory, in our previous study, also agrees well with this STS data.19 This higher number of empty states with antibonding orbitals would assist in moving TMP adsorbates more frequently at lower bias conditions, as shown in Figure 7. Peaks did not appear at lower bias conditions in the STS analysis as expected, which could be directly connected to 4985


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Figure 9. STM images of the TMP adsorbates in the self-assembled pyrrole fence obtained at different tunneling current conditions obtained by scanning over the same position. Sample bias is fixed at +2.0 V.

Table 2. Relationship of Tunneling Current Condition and TMP Movement TMP movement frequency (mov/min)

free-to-move TMP adsorbates (%)

It (nA)

VS = 2.5 V

VS = 2.0 V

VS = 1.5 V

VS = 2.5 V

VS = 2.0 V

VS = 1.5 V

0.10 0.15 0.20 0.25 0.30 0.35 0.40

19.13 14.21 12.52 19.12 17.49 13.11 14.75

32.32 34.52 33.93 33.34 29.58 33.61 38.92

48.23 68.51 56.83 60.65 53.56 52.25

27 24 24 31 33 30 24

44 54 51 51 50 48 60

56 71 55 57 66 64

molecular corral respond to the changes in the bias voltage conditions, it is vital to find a method to control the movement of TMP adsorbates by changing the electrostatic potential. This will bring another step closer to fabrication of a molecular level switch which is the end purpose of this research project.

same pattern as the tunneling current is increased. It is possible to understand that the same set of TMP adsorbates has been involved in the movement and additionally some TMP adsorbates have migrated from outside into the scanning area. From these obtained data, it can be concluded that there is no significant effect on the movement process of TMP by the tunneling current and thus from the tip−sample distance. Even though the tip can be influential on loosely bonded TMP adsorbates, the bond strength is not affected by the tip itself, and hence the TMP movement frequency. Due to TMP−Si interaction and the TMP dynamic motion, this molecular arrangement also acts as a flexible molecule-to-substrate electron donating system which will be influential for the future molecular electronic device fabrication. As it can be understood that TMP adsorbates enclosed in a pyrrole

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CONCLUSION Self-assembled molecular corral structures were obtained when the Si(111)-(7 × 7) surface was exposed to pyrrole gas. The TMP molecules were then adsorbed and enclosed in these molecular corrals. The dynamic motion of these TMP adsorbates was observed via STM under different scanning conditions, and it was noted that the TMP movement frequency and the number of free-to-move TMP adsorbates increased with the decrement of the sample bias condition 4986



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11474. Sequential STM images obtained by keeping It = 0.10 nA and VS = +2.5, +2.0, +1.5, and +1.0 V; sequential STM images obtained by keeping VS = +2.5 V and It = 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40 nA (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Masaru Shimomura: 0000-0002-6862-1781 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. H.D.S. and M.S. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this research was supported by JSPS KAKENHI, Grant Numbers 26105007 and 25390077.

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ABBREVIATIONS STM, scanning tunneling microscopy; STS, scanning tunneling spectroscopy; TMP, trimethylphosphine; Vs, sample bias voltage; It, tunneling current

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REFERENCES

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Figure 10. Relationship of the TMP movement frequency and free-tomove TMP adsorbates with tunneling current conditions at (a) Vs = +2.5 V, (b) Vs = +2.0 V, and (c) Vs = +1.5 V.

under a fixed tunneling current. At lower sample bias conditions, electrons can tunnel into the antibonding orbitals of the Si−P bond which lie closer to the conduction band, weakening the bond and driving the TMP movement. However, the tunneling current conditions and the TMP movement frequency did not indicate any relationship, suggesting that the TMP movement is not affected by the tip−sample distance. 4987


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