Lorenzo Poggini
email: lorenzo.poggini@icmcb.cnrs.fr

Mathieu Gonidec
email: mathieu.gonidec@icmcb.cnrs.fr

Ram Kumar

Lorenzo Squillantini

Gilles Pecastaings

Andrea Caneschi

Patrick Rosa

Temperature-induced transport changes in molecular junctions based on a Spin Crossover complex

This work describes the study of molecular junctions embedding the spin crossover complex [Fe(H2B(pz)2)2(phen)] as an active switchable thin film. In those junctions we observed that the spin state conversion causes a gradual increase in the current density, may be due to a crossover from direct tunneling to multi-step hopping.

Molecular-based materials start to catch the attention of the research community as a potential alternative to conventional inorganic semi-conductor materials, due to the possibility to incorporate them in advanced materials that can be easily processed, that are lightweight, and that can be incorporated in flexible devices. Switchable materials, i.e. materials able to reversibly switch their physical properties upon application of external stimuli (temperature, lightirradiation, applied magnetic and electric fields, pressure, etc.) are of relevant importance in material science for their potential technological applications as active material in functional devices. [1][2][3][4][5][6][7] Among them, spin crossover (SCO) complexes, in which the spin state of a metal ion can be switched between two distinct magnetic states (low-spin, LS, and high-spin, HS), are particularly appealing molecular materials for the preparation of molecular switches with reversible magnetic and electrical response. 5 Only few studies have been reported so far on the integration of SCO materials in electrical and electromechanical devices, 5,8 but one of the main challenges has resided in the difficulties of obtaining high-quality thin films. Due to those drastic limitations, and notably to the difficulty to prepare nonshort-circuited, high-quality devices, most of the studies to date have focused on rather thick films that operate in the hopping regime.

We believe, however, that there are very interesting phenomena to study on ultra-thin films (i.e. films which are at most a few nm thick) for which the tunnelling regime is usually expected be preponderant. 9 Most interestingly, unlike hopping, tunnelling characteristics should be temperature independent and would thus provide a flat baseline for SCO-induced current switching. Beyond simple electronic effects, interfacial phenomena are extremely relevant for spintronics, and in this context it is important to increase our knowledge of molecularscale phenomena instead of bulk behaviours. 10 In a previous work 7 we studied the tunnelling properties of vertical junctions incorporating very thin films of a SCO complex, [Fe(HB(trz)3)2] (HB(trz)3 = tris(1H-1,2,4-triazol-1yl)borohydride), presenting a remarkably abrupt spin crossover slightly above room temperature. We had selected this compound as an experimentally easy accessible starting point, as it only required the capability to alter the temperature of the sample in a narrow range above room temperature. In those experiments, we showed that the temperature-induced spin transition of the compound translated in a major change in the current density flowing through Au TS //[Fe(HB(trz)3)2]//EGaIn junctions. We observed an increase in log |J| of two orders of magnitude upon converting from the LS to the HS state, which we rationalized by a theoretical approach. 7 These interesting results gave a first sight at the promising direct coupling between temperature-induced SCO and tunnelling in ultra-thin films. Nevertheless, this is still a budding field and much more experimental data are needed to fully understand it. In particular, our previous experiments were performed above room temperature (and thus above the freezing point of the EGaIn electrode) with thin films exhibiting a very abrupt SCO behaviour but complicated properties depending on the hydration state of the films. 11,12 Since most compounds exhibit SCO below room temperature, we believe that it is therefore important to study model high-quality SCOjunctions operating below room temperature, and more important below the freezing point of the liquid EGaIn top electrode. 13 (A) We patterned templatestripped gold (Au TS ) stripes as the bottom electrodes by thermal evaporation of gold on an ultra-flat silicon wafer through a shadow mask. To allow for an easy pealing of the stripes, we modified the SiOx, selectively, by vapour silanization with a fluorosilane. We cast a UV-curable optical adhesive (OA) to glue the M TS stripes to a glass chip, and after UV exposure (C) we cleaved the Au/adhesive/glass composite from the wafer to obtain the array of electrodes (D). After thermal evaporation of 1 over the substrate, we filled a PDMS microchannel with EGaIn and brought it in contact with the array (E) to complete the junctions that could then be connected to the electrometer (F) and mounted inside a Linkam temperature control stage.

In this context, we have shown that the ferrous complex [Fe(H2B(pz)2)2(phen)] (pz = pyrazol-1-yl, phen = 1,10phenanthroline) 1 is an ideal target, since it can be deposited as very smooth high-quality ultra-thin films by thermal evaporation while still maintaining its actuating properties. 14 This is a very well-known compound for which the synthesis, 15 structural characterizations, 16 magnetic and photomagnetic properties together with Raman and Mössbauer spectroscopies in bulk [17][18][19] have been extensively described. We and others elsewhere showed later on the feasibility of obtaining thin films by sublimation of 1 and reported their properties. 14,[20][21][22][23] In this paper, we study the modulation in tunnelling current densities across Au TS //1//EGaIn junctions as a function of temperature and film thickness. We show that, as expected, there is a significant change in current density with temperature due to the switching between HS and LS states. More precisely, we show that the system switches gradually from a low conductance state at low temperature (when the compound is in the LS state) to a high conductance state at high temperature (when the compound is in the HS state). We rationalize this behaviour by means of gas-phase DFT calculations. Interestingly, we detect no specific discontinuity around the freezing temperature of the EGaIn alloy, which could have modified the behaviour of the device due to the mechanical constraints on the SCO thin films, since SCO compounds are known to be sensitive to pressure. 24 To measure molecular junctions incorporating 1 as the active switchable layer, we used as top electrode the eutectic alloy of indium and gallium (EGaIn), which acts as a soft, conformal, non-damaging top electrode as shown previously by us and others. 7,13,25,26 In order to study the junctions characteristics at low temperature, we used arrays of crossbar geometries as described by Nijhuis et al., 13 in which the active compound is evaporated on top of an array of templatestripped metal tracks, then covered with a top electrode consisting in an open microchannel made of polydimethylsiloxane (PDMS) filled with EGaIn (see Fig. 1). We then mounted the devices inside a Linkam stage equipped with electrical feedthrough connections to control the temperature while performing J-V measurements. We used this method to construct robust arrays of evaporated tunnel junctions and to study their J-V characteristics over a temperature range that was broad enough to encompass the complete thermal SCO temperature window (i.e. 100-300 K).

As it was already pointed out by Nijhuis et al., 13 it is critical that the metal tracks be fully embedded in the optical adhesive to prevent short-circuits due to micro-fabrication defects at the edges of the template-stripped metal (Fig. 1). An important source of short-circuits in molecular junctions is the roughness of the metal films and the inhomogeneities and pinholes in the molecular film. However, template-stripping provides highquality metal-films with a very low roughness, and it is well known that [Fe(H2B(pz)2)2(phen)] forms very smooth films down to a few nanometres in thickness on gold. 14 We checked the quality of our evaporation process, and calibrated our tooling factor, by atomic force microscopy (AFM). We first evaporated a 20 nm thick film of compound 1 on an ultra-flat Si wafer through a porous mask, and we characterized the resulting film by AFM. As expected, the resulting film (see Figure S1) is very smooth, and we used those data to correct the experimental thicknesses by calibrating the equipment's tooling factor, considering the error due to surface roughness. We then studied the morphology of a 10 nm thin film of 1 evaporated on a template-stripped gold substrate. On a 1×1 μm scale, the morphology of pristine Au TS (see Figure S2) and of a thin film of 1 (see Figure S3) were quite different, but both exhibited a very low surface roughness (below 1 nm). The high quality of those films were also demonstrated by comparing their long range morphology to that of the ultraflat Au TS substrates: on a 20×20 μm scale both surfaces appear almost identical, without major defects or pinholes (see Figure S4).

We prepared thin films of 1 of different thicknesses by thermal evaporation over template-stripped gold substrates. After taking into account the correction due to the tooling factor, the films we prepared had nominal thicknesses of 5.6±0.4, 11.2±0.8 and 16.7±1.1 nm. We performed temperature-dependent measurements from 100 to 300 K on the 5.6 and 16.7 nm films, and we collected a more extensive amount of room-temperature data on the 11.2 nm film by using a standard conical-tip methodology.

The 11.2 nm film (35 junctions, 350 traces, on a single film) presented log|J| values of -6.50 ± 0.09 and -6.39 ± 0.08 at +1.0 and -1.0 V, respectively (see Figure S5). The standard deviations of those measurements were smaller than that of similar measurements performed on Au TS //[Fe(HB(trz)3)2]//EGaIn junctions, 7 and for other junctions embedding self-assembled monolayers (SAMs) of small molecules. [27][28][29] It is also noteworthy that we got 100% yield in working junctions. These unusually narrow standard deviations and the absence of shorted junctions are remarkable and can be seen as a confirmation of the high quality of the thermally evaporated film of 1 on Au TS . On the contrary, attempts to perform measurements with [Fe(H2B(pz)2)2(bipy)] (bipy = 2,2'-bipyridine), a closely related compound that is well known to form rough films that are not necessarily continuous, 14 only yielded shorted junctions.

We then performed temperature dependent measurements on the 5.6 and 16.7 nm films. Thanks to the relatively high geometrical contact area in the crossbar junctions (about 1000×50 µm 2 ), we were able to measure very low current densities (down to log |J|≈ -9.5) with a reasonable signal-to-noise ratio, which constitutes a significant improvement in respect to our previous study. 7 For both film thicknesses, we measured full J/V traces between -1 and +1 V at different temperatures between 300 and 100 K (corresponding to the temperatures in which the SCO complex is in either HS or LS states as determined by magnetometry and by X-Ray Absorption Spectroscopy, XAS, in previous works 14,22,30,31 ). The log|J|/V curves measured between +1 V and -1V present the usual symmetrical V-shaped characteristics of non-rectifying junctions (see Figure 2). In both cases, upon increasing the temperature, the current density increased, with overall changes in the logarithm of current density, Δlog |J| measured at ± 1V, of about 0.6 and 1 for the 5.6 and 16.7 nm samples, respectively. Due to the rather gradual SCO behaviour measured by XAS usually displayed in thin films of 1 as compared to the bulk, it is not surprising that the current density changes very gradually with temperature. 22,30 The trend in current density as a function of temperature is not very different from what was previously observed for the same compound with evaporated junctions using Al and ITO electrodes and with a higher thickness (30 nm). 4 Nevertheless the shape of the current vs. T curve does not match directly that of the SCO curve measured on bulk and thick film samples by standard magnetometry, 14 but seems to be in line with what was found for a sub-monolayer coverage. 22 The discrepancy with bulk samples is perhaps not surprising, as it has been shown that for the first layer on Au(111) single crystal, ca. 80% of the molecules do not undergo spin crossover, and that a portion of those surface bound [Fe(H2B(pz)2)2phen] decompose into [Fe(H2B(pz)2)2] and free phen moieties. 22 Considering the thickness of our films, those interfacial species should constitute only a small fraction (around 4-12%) of the total amount of compound 1 present in the films. Remarkably, even at room temperature, the log |J| vs T curves (both at +1 V) do not seem to reach a saturation value, and it is offset significantly towards high temperatures when compared to bulk susceptibility measurements.

Those unexpected features could be related to nonanticipated behaviour of compound 1 in our junctions (stress induced by the top electrode, pinning of the spin state due to the interactions with the bottom substrate, electrostriction…) or could be due to a more complex phenomenon, such as a change of transport mechanism with temperature. In particular, the latter situation would be quite complex as it could involve the combined effect of the change in spin state and the change in thermal energy. To try to unravel those mechanistic aspects, we compared the current densities in the Au TS //1//EGaIn junctions as a function of thickness. As shown on Figure 3, log |J| appears to vary linearly with the thickness of the molecular film at both positive and negative biases, and the J/V curves are shifted towards higher current densities with decreasing film thicknesses. This result could point at direct tunnelling as the dominating mechanism at room temperature, for which the thickness dependence is often written in a simplified manner as:

𝐽(𝑉) = 𝐽 0 (𝑉)𝑒 -𝛽𝑑
where J0 (A*cm -2 ) is a constant that depends on many variables of the system including the contact resistance, the nature of the interfaces between the molecular layer and the electrodes and the properties of the molecules, while  is the tunneling decay coefficient that depends essentially on the properties of the molecules (e.g. the frontier orbital energies). Nevertheless, the decay coefficients , resulting from the fitted experimental data, are very small (of the order of 0.6 nm -1 as opposed to the usual values of the order of 10 nm -1 for many compounds). This suggests that if the mechanism truly was tunnelling, the barrier height (that results from the difference in energy between the work functions of the electrodes and the frontier molecular orbitals of the compound) would be extremely small. In fact this  value is more in line with values measured for molecular junctions exhibiting multi-step hopping. 9 To shed some light on the small  value, we performed gas phase DFT calculations (see details in the SI), to estimate the frontier orbital energies of compound 1 in both HS and LS states. The gas-phase calculations are of course an oversimplification of the system that does not take into account the interactions with the surface and between neighbouring molecules, but we have found, with other compounds and as pointed out in our previous study, 7 that they provide a reasonable ground to discuss the electronic configuration of our junctions. As it is often the case with molecular systems, the LUMOs lie very far from the work functions of the electrodes (we take φAu = 5.1 eV and φEGaIn = 4.2 eV) for both HS and LS states, and hole transport should dominate the behaviour of the junctions. The relevant frontier orbitals are therefore the HOMOs with estimated orbital energies of about -5.3 eV and -4.9 eV for HS and LS, respectively. With those data, we constructed the molecular orbital diagram shown on Figure 4. Based on this diagram, the HOMO energy in the LS configuration should be quite close to the fermi energy of the gold electrode but should still lie below the fermi energies of both top and bottom electrodes. It follows that direct hole tunnelling is likely to be the dominating transport mechanism below the T1/2 of compound 1 (i.e. below ca. 150 K). For the HS state, however, the HOMO energy (-4.9 eV) lies between the Fermi energies of the top and bottom electrodes (-5.1 and -4.2 eV, respectively) while staying rather close to the work function of gold. This could support that, upon SCO, the system could exhibit a crossover from direct tunnelling to multi-step hopping. 9,32 We note that the energy difference between φAu and the HOMO energy of compound 1 is rather small in both cases (0.2 eV for both HS and LS) and one should therefore be very cautious when assigning the precise nature of the transport mechanisms involved in those junctions. Nevertheless, such a crossover from a temperature-independent tunnelling regime to a thermallyactivated one would account well for the relatively stable current densities we observed below 150 K, and the absence of saturation of the tunnelling current density even at room temperature.

Conclusions

In conclusion, we have studied ultra-thin molecular junctions incorporating high-quality films of the [Fe(H2B(pz)2)2(phen)] spin-crossover complex and shown that such junctions present a significant variation in current density by temperature. To perform this study, we used EGaIn-based crossbar tunnel junctions, a non-damaging technique that we recently employed to study another SCO based tunnel junction presenting a very abrupt current switching. 7 In particular, we wanted to check in this study whether junctions embedding compound 1 (that switches spin states about 160 K) would not be quenched by the freezing of the liquid metal top electrode (at 288 K). Instead, we show that the resulting Au TS //1//EGaIn junctions present a change in current density of up to one order of magnitude with temperature. This change occurred in a very broad range of temperature (100-300 K), which is in line with what would be expected for SCO complexes exhibiting a gradual conversion, as was shown for ultrathin films of complex 1. This is a very important result as it shows that despite being below the freezing point of EGaIn, the device still presents spin-state switching that translates in a significant change in current density. Thanks to DFT calculations, we can propose a rationalization of the behaviour of the devices and it seems as though the spin conversion is accompanied by a crossover from direct tunneling to a temperature activated multi-step hopping regime. Such crossovers are well known in oligo-aromatic molecular wires when varying the length of the compound. 9,32 In the present case, however, the switchable nature of the SCO complexes seems to allow to induce it in a single system by a change of temperature. This result, if confirmed, would open the door to interesting molecular switches in which the compounds could be engineered to favour such an inter-mechanism crossing, and that could be triggered by light irradiation thanks to the LIESST effect.

Figure 1 .

 1 Figure 1. Fabrication of SCO-based tunneling junctions for temperature-dependent measurements based on the design of Nijhuis et al.13 (A) We patterned templatestripped gold (Au TS ) stripes as the bottom electrodes by thermal evaporation of gold on an ultra-flat silicon wafer through a shadow mask. To allow for an easy pealing of the stripes, we modified the SiOx, selectively, by vapour silanization with a fluorosilane. We cast a UV-curable optical adhesive (OA) to glue the M TS stripes to a glass chip, and after UV exposure (C) we cleaved the Au/adhesive/glass composite from the wafer to obtain the array of electrodes (D). After thermal evaporation of 1 over the substrate, we filled a PDMS microchannel with EGaIn and brought it in contact with the array (E) to complete the junctions that could then be connected to the electrometer (F) and mounted inside a Linkam temperature control stage.

Figure 2 .

 2 Figure 2. (a,c) Plot log |J| vs V at different temperatures and b,d) plot of log |J| at ±1V reported as a function of T for junctions incorporating 5.6 nm (a,b) and 16.7 nm (c,d) films of 1, respectively.

Figure 3 .

 3 Figure 3. a) Plots of log |J| vs. V at room temperature for junctions of 1 with different film thickness, and b) thickness dependence of the log |J| values at ± 1V. The continuous lines are the best linear fits to the data.

Figure 4 .

 4 Figure 4. Electronic configuration of the junctions in open circuit for both HS (a) and LS (b) states, respectively.

Acknowledgements

This work was supported by the French National Research Agency (ANR) Investment for the Future Programme IdEx Bordeaux (ANR-10-IDEX-03-02) and the MOLSPIN COST action CA15128. Computer time for this study was provided by the computing facilities MCIA (Mésocentre de Calcul Intensif Aquitain) of the Université de Bordeaux and of the Université de Pau et des Pays de l'Adour. We also acknowledge the LCPO laboratory and all the ICMCB staff for their support with the measurements.

Conflicts of interest

There are no conflicts to declare.


Indra Ngurah

Er

Deep Kamal

Jean-Marie Singh

Bonnin

Ngurah Indra
email: indra@unud.ac.id

Singh

Jean-Marie Bonnin
email: jean-marie.bonnin@irisa.fr

Contact Strategy for VDTN Data Collection in Smart Cities

Keywords: Contact Strategy, Data Collection, Smart Cities, DC4LED, Hierarchical VDTN Routing

come   L'archive ouverte pluridisciplinaire

I. INTRODUCTION

It is forecasted that more than 30 billion connected devices will be installed worldwide by the year 2020, and it will be more than doubled that number in the year 2025 [START_REF]IoT: number of connected devices worldwide 2012-2025[END_REF]. Most of those devices will provide data for smart cities and its citizen. Consequently, a large amount of data need to be collected and to be delivered to each destination and then processed by corresponding applications. One such application is delay-tolerant applications, where it can be feed by data which does not need to arrive instantly, i.e., communication latency in order of minutes or even hours is adequate. Some examples of such applications are the environmental monitoring, smart metering [START_REF] Santos | PortoLivingLab: An IoT-Based Sensing Platform for Smart Cities[END_REF], photos reporting of road degradation, etc.

On the other hand, the era of connected-vehicles is starting to become a reality. Vehicles will have the capability to exchange information between them, as well as with their surrounding environment, with vehicle-toeverything (V2X) communications technology. They can also have the connectivity to the internet by utilizing current and future radio access networks. Decision makers around the world will soon make such capabilities compulsory for vehicles [START_REF] Machardy | V2X Access Technologies: Regulation, Research, and Remaining Challenges[END_REF], which further elevates the possibility for vehicles to play an integral part in smart cities ecosystems.

Our previous work in [START_REF] Indra Er | DC4LED: A Hierarchical VDTN Routing for Data Collection in Smart Cities[END_REF] put forward an idea of a vehicle-based data collection architecture for smart cities, as illustrated in fig. 1. It is designed to offload data for delay-tolerant applications, utilizing V2X capabilities. In the architecture, data need to be collected from several wireless sensors in a smart city and delivered to associated application servers. The system utilizes Vehicular Delay-Tolerant Networks (VDTN) with its store-carry-forward mechanism to gather and deliver data to one of the wireless Internet Point-of-Presence (PoP) available in the city. Furthermore, we proposed and evaluated DC4LED (Data Collection for Low Energy Devices): a hierarchical VDTN routing, which sensibly takes into account several common features of mobility in smart cities (e.g., buses, taxis, and cars), and hierarchically defines their role in forwarding the data. A more detailed description of our previous work is presented in the next section.

An interesting result from our previous work is shown in fig. 2. It shows dropped messages percentage and its distribution among nodes, with increases in cars density. It shows that the percentage of dropped messages decrease with the increase in available mobility. It also points out that most of the drops happen in sensors and that drops decrease as more cars are in proximity to gather messages. Note that the number of dropped messages are in decimal as each value was averaged over ten simulation runs with different mobility patterns. The fact that most drops were occurring in sensors motivate us to investigate further into the dynamic of communications contact between sensors and vehicles. Thus, this article aims to put forward We organize the rest of this paper as follows. Section II describes our previous work which motivates this study. Section III presents our idea on the contact strategy for wireless sensor. Lastly, conclusions and future works are provided in Section IV.

II. PREVIOUS WORK

As introduced above, our previous work in [START_REF] Indra Er | DC4LED: A Hierarchical VDTN Routing for Data Collection in Smart Cities[END_REF] proposed a vehicle-based data collection architecture for smart cities. We focus on data collection in cities using VDTN routing. The data collection chain in a city starts from sensors and then the data is routed using cars, taxis, and buses, to an Internet PoP and then finally to Server. Our concept of DC4LED: a hierarchical VDTN routing to accommodate the data collection process is illustrated in fig. 3. The hierarchy of its forwarding decisions is based on the reliability and capability of each category of nodes to deliver data to the server. The idea is to statistically assign a level to the nodes in the city, instead of having complex routing decisions and metrics. Note that a node only forward data to another node with a superior hierarchical level. For example, a car will forward data to a taxi, a bus, or directly to an internet PoP, but not to another car. A taxi will forward to a bus, or directly to an internet PoP, but not to another taxi or a car, and so on. We evaluate the performance of DC4LED Routing algorithm by using the Opportunistic Networking Environment (ONE) simulator [START_REF] Keranen | The ONE Simulator for DTN Protocol Evaluation[END_REF] [START_REF] Dede | Simulating Opportunistic Networks: Survey and Future Directions[END_REF]. The simulation scenario envisions an air pollution monitoring in the city of Helsinki, where 37 wireless sensors were placed almost evenly in an area of about 9 km 2 . Table I provides latency and percentage of dropped messages at each sensor. The statistical snapshots were averaged over all mobility scenarios. Fig. 4 shows average latency values between sensor locations, where generally most sensors located in the outer part of the city had higher latency compared to ones installed in the inner area, with the exception for sensors that had proximity to a bus route. The latency value was as high as 119.12 minutes in one part of the city and as low as 3.28 minutes in another location. There are also some sensors located in the inner city which did not encounter enough mobility while also out-of-range from the nearest bus route, which caused high latency. It points out the disparity in mobility which affects Key Performance Indicators (KPI) in different parts of the city, and that each sensor may have to adapt to different data pickup patterns. Fig. 5 illustrates the average percentage of dropped messages for all mobility scenarios, and it gives a contrast of percentages between sensors in different locations. The drops can be as high as 51% in one location and can be none in some other areas. Sensors in this low mobility places will need to have other strategies or even other connectivity such as LPWAN. Further observation showed that drops are primarily caused by messages reaching its Time-to-Live (TTL) of 5 hours. In our scenario, where one message of size 1 KB is generated every 5 minutes, 64 KB of buffer size in each sensor proved to be sufficient to hold messages for more than 5 hours. This fact shows the relation between the required sensors buffer size and the TTL parameter which reflect the user-defined data usefulness period.

The geographical mapping reveals the disparity of vehicular mobility experienced by sensors related to location, which emphasizes the requirement for each sensor to adapt to different data pick up patterns in the city. This finding is our motivation to investigate further the contact strategy for wireless sensors in VDTN data collection.

III. CONTACT STRATEGY FOR WIRELESS SENSOR

Different phases of data collection in wireless sensor networks with mobile elements have been described in [START_REF] Di Francesco | Data Collection in Wireless Sensor Networks with Mobile Elements: A Survey[END_REF], where contact can be defined as a condition when two nodes or more are in communications range to one another, thus, are able to exchange data. In our proposed architecture, we consider that the vehicular networks should be agnostic in term of the contact mechanism. Therefore, the contact strategy should be managed by the sensor itself. Fig. 6 shows an ideal contact scenario between a sensor and a vehicle. During contact time, a discovery process will take place, which allows sensors to detect the presence of vehicles before starting to exchange data. In an ideal scenario of mobility shown in fig. 6, where we assume that the vehicle's speed Vs (in m/s) is constant, we can simply relate it to the mutual communication range R (in metres) and find the contact duration Cd (in seconds) If we consider periodic discovery time of t (in seconds), then we can calculate the highest possible data transfer windows during each contact Tw as Fig. 7 illustrates relations between vehicle's speed, data transfer windows, and different periodic discovery time. In the calculation, we assume a maximum radio communications range of 10 m between the sensor and the vehicle, which make up R value of 20 m. The vehicle's speed is increased from 30 to 100 km/h, which corresponds to values of 8.33 to 27.78 m/s. Results for three different t values; 0.1 s, 0.2 s, and 0.5 s, are then presented. The graph shows that for a certain value of R

𝑇 ( = ) * + (2)
and t, the number of data transfer windows decreases exponentially with the increases in vehicle's speed. Furthermore, graph comparison between different t values emphasizes the decreasing number of data transfer windows as the discovery period increases. Hence, the availability of a higher data transfer windows also means more data can be offloaded during contact.

In communications between low energy devices, such as wireless sensors, and moving vehicles; a large portion of energy is consumed during discovery periods [START_REF] Casilari | Modeling of Current Consumption in 802.15.4/ZigBee Sensor Motes[END_REF], while the available contacts cannot be guaranteed. As shown in the previous section, the average availability of vehicles to pick-up data can be in the order of several minutes, or even hours depending on location. Thus, an adaptive contact strategy, particularly in the discovery process, need to be devised to reduce energy consumption. In fig. 8 we put forward a contact strategy which takes into account the buffer occupancy of the sensor. The logic is that when a sensor does not have data in its buffer to send, then the discovery process does not need to be initiated. On the other hand, when the buffer occupancy is high, a sensor needs to initiate the highest possible frequency of discovery (i.e. the lowest possible t). Therefore, sensors with high buffer occupancy will need more data transfer window, while sensors with low buffer occupancy will need less and can reduce their energy consumption, referring back to an example shown in fig. 7.

IV. CONCLUSION AND FUTURE WORKS

This paper has described a preliminary study on a contact strategy for wireless sensors data collection utilizing VDTN in smart cities. The study emanated from our previous work which showed the diversity of data pick-up pattern by vehicles in the city. Therefore, sensors need to adapt its discovery period to reduce wasteful energy consumption. We then discussed the relation between mutual communications range, vehicle's speed, and periodic discovery time. Furthermore, we proposed sensor's buffer occupancy value as a measure for the adaptation.

Our work is continuing in the formulation of an adaptive contact strategy through an empirical study based on findings from our simulation of VDTN data collection. There are also possibilities to include other parameters such as the average data latency in each sensor, the average contact duration, etc., to the strategy.

Fig. 1 .

 1 Fig. 1. Vehicle-Based Data Collection Architecture for Smart Cities

  the parameters and values used in the simulation. Simulation results depicted in fig. 4 and fig. 5 give insight into the dynamics of data collections in the city, where colors and the size of blobs emphasize values of average

Fig. 3 .Fig. 2 .

 32 Fig. 3. The Concept of DC4LED: A Hierarchical VDTN Routing

Fig. 4 .Fig. 5 .

 45 Fig. 4. Mapping of the average latency

Fig. 7 .

 7 Fig. 7. The Dynamic of Data Transfer Window

TABLE I SIMULATION

 I PARAMETERS AND VALUES

ACKNOWLEDGMENT This work is supported by a Doctoral Scholarship from the Indonesian Endowment Fund for Education (LPDP) of the Indonesian Ministry of Finance, under Contract Number PRJ-43/LPDP.3/2016.