N Gogneau
email: noelle.gogneau@lpn.cnrs.fr

N Jamond

P Chrétien

F Houzé

E Lefeuvre

M Tchernycheva
email: maria.tchernycheva@u-psud.fr

From Single III-Nitride Nanowires to Piezoelectric Generators: New Route for Powering Nomad Electronics

Keywords: GaN nanowires, Piezoelectric properties, Energy harvesting, Nanogenerators, Piezogenerators

Ambient energy harvesting using piezoelectric nanomaterials is today considered as a promising way to supply microelectronic devices. Since the first demonstration of electrical energy generation from piezoelectric semiconductor nanowires in 2006, the piezoelectric response of 1D-nanostructures and the development of nanowirebased piezogenerators have become a hot topic in nanoscience. After several years of intense research on ZnO nanowires, III-nitride nanomaterials have started to be explored thanks to their high piezoelectric coefficients and their strong piezogeneration response. This review describes the present status of the field of piezoelectric energy generation with nitride nanowires. After presenting the main motivations and a general overview of the domain, a short description of the main properties of III-nitride nanomaterials is given. Then we review the piezoelectric responses of III-N nanowires and the specificities of the piezo-generation mechanism in these nanostructures. Finally, the design and performance of the macroscopic piezogenerators based on nitride nanowire arrays are described, showing the promise of III-nitride nanowires for ultra-compact and efficient piezoelectric generators.

Introduction

Portable electronic equipment such as biomedical devices, portable multimedia, distributed sensor networks or mobile communication are omnipresent in our daily life and the question concerning their autonomy is a key societal challenge. Electronic devices have known a tremendous progress in the past decades with the development of systems at micrometer and even nanometer scale. The device miniaturization in particular results in the reduction of their energy consumption going down to milliWatt and even microWatt. Despite the recent progress in the development of batteries with high energy density [Arma2009;Mill2008] as well as of nanobatteries, these sources are not ideally suited to supply electronic microdevices since they present limitations such as complex integration, limited capacity and considerable cost. In this context, new competitive ultra-compact and integrable electrical sources should be developed generating sufficient power to supply the microdevices without increasing microsystem size and adding weight.

Energy harvesting, i.e. conversion of the ambient energy found in the vicinity of the device into usable electrical energy, appears as the most promising way for powering wireless microdevices. Among the alternative sustainable energy resources, the mechanical deformations and vibrations present the advantages to be ubiquitously available at all time and more accessible than solar and thermal energy. The vibration energy may have various origins such as body movements, sound vibrations, hydraulic movements, wind, friction…. Harvesting of this energy through the use of piezoelectric materials has received these last years a great attention due to the abilities of these materials to convert mechanical deformation directly into electrical energy. Piezoelectric energy harvesting is not a new concept. It has been practiced for decades at the macroscopic scale and commercialized. The most common application of piezoelectric materials is the fabrication of sensors and actuators, however their use as electrical generators remains until now rather limited (the best-known applications are the quartz used in watches or the electric cigarette lighter). Indeed, the power output from conventional bulk piezoelectric ceramics is low. In addition, bulk materials are not well suited for energy harvesting applications since they are massive, not easily integrable and their sensitivity to ambient mechanical vibrations is very weak. To increase the sensitivity to the applied force and thus the conversion efficiency, microstructured piezo-composite materials such as for example 1-3 connected ceramic-polymer composites have been developed [Chan2005]. However, the fiber diameter in these materials (tens to hundreds microns) remained rather big. With the development of nanotechnologies, it has been shown that further downscaling of the fiber dimensions, i.e. their replacement by nanowires (NWs), can significantly enhance the conversion efficiency.

Semiconductor nanowires presenting piezoelectric properties have recently emerged as excellent candidates to fabricate novel ultra-compact and efficient piezoelectric generators. Due to their specific structural properties, these 1D-nanostructures can enhance the piezoelectric properties of the material compared to its bulk or 2D form. In particular, the NWs present superior mechanical properties. Due to the structural perfection of NWs, resulting in absence of defects, such as dislocations, they can be subject to large elastic deformation without plastic deformation or fracture [Yang2010]. They are also characterized by higher flexibility, higher robustness and higher resistance to fatigue, which extend the operational lifetime of the piezogenerator. The NWs can be bended under an extremely small applied force due to the small diameter and the large aspect ratio characterizing these 1D-nanostructures. Hence, it is possible to generate a significant strain in the NWs under pico/nano-Newton applied forces [Zhou2012]. Finally, as illustrated in Table 1, quasi-1D geometry results in an enhancement of the piezoelectric coefficients in comparison with their bulk counterparts, due to surface effects [Zhao2004;Agra2011;Mina2012]. The first electricity generation from the mechanical deformation of ZnO NWs has been demonstrated by the group of Prof. Wang from the Georgia Institute of Technology in USA [Wang2006]. Since this first demonstration, several 1D-nanostructures including nanostructured conventional piezoceramics have been studied, such as CdS [Lin2008], CdSe [Zhou2012b], PZT [Chen12], BaTiO 3 [Wang2010], etc. Despite their strong piezoelectric response [Bern1997], IIInitride nanostructures have been much less studied compared to ZnO. These materials, crystallizing in wurtzite phase with hexagonal structure, are characterized by a strong internal electric field, which results from the combination of the spontaneous and piezoelectric polarizations. The first electric generation resulting from the deformation of a GaN nanorod has been evidenced in 2007 by Su et al. [Su2007]. The investigation of piezoelectric properties of GaN nanowires was then continued by different groups [Xu2010; Huan2010b; Xu2011; Gogn2014]. Other nitride NW materials than GaN were investigated, namely AlN, AlGaN [Wang2010b] and InN [Ku2012;Huan2010]. Up to now, the best mechanical-electrical conversion from NWs in terms of the output voltage has been obtained with GaN and InN NWs, with a maximum generated voltage per nanowire of about 440 mV [Gogn2014] and 1 V [Huan2010], respectively. These output voltages measured from single piezoelectric III-N NWs are larger than those for other piezoelectric semiconductors, and in particular ZnO NWs, which is today the most widely used material in nanostructure-based piezo-generation and whose output voltages do not exceed 45 mV [Gao2007] and 80 mV [Lu2009] for respectively n-doped and pdoped materials.

It should be noted that semiconductor NWs such as ZnO or nitrides differ from conventional piezoceramics by the presence of free charges (controlled by doping), which impact the piezoconversion. From the theoretical viewpoint, Finite Element Method is usually used to analyze piezoconversion with semiconductor NWs and in particular to predict the electric potential resulting from their deformation (see for example [Hinc2014]).

The studies of piezoelectric properties of individual semiconductor NWs have naturally lead to the development of macroscopic NW-based piezo-generators. To avoid confusion existing in the literature, in this review we define a "nanogenerator" as a single nanowire producing electricity via piezo effect, while we call "nanowire-based piezogenerator" a macroscopic device based on a nanowire array. The first prototype of an integrated piezogenerator realized by Prof. Wang's group was based on a single layer of ZnO NWs and exhibited an operational power output of approximately 2.5 nW/cm 2 [Wang2007]. Subsequent realizations have significantly improved the performance of the ZnO-based piezo-generators. Three different configurations have been introduced: the Lateral nanowire Integrated Nanogenerator (LING) [Xu2010], the Nanocomposite Electrical Generators (NEG) [Mome2010] and the Vertical nanowire Integrated Nanogenerator (VING) [Xu2008]. The interest of the use of flexible substrates to fabricate nanogenerators has been first evidenced by a new prototype based on cyclic stretching/releasing of a single piezoelectric wire packaged on a flexible substrate. Such nanogenerator configuration producing an AC output voltage of around 50 mV has been reported as very attractive for harvesting biomechanical energy [Yang2009]. The advantage of this flexible support has been combined to the VING configuration in order to increase the generator performance. An output power density of the order of several hundred of mW/cm 3 was reported [Zhu2012].

A fundamental difference should be noted between NW piezo-generators and piezoelectric Micro Electro Mechanical Systems (MEMS). MEMS operate in a resonant mode (with a typical resonance frequency in the MHz range), which is not efficient for energy harvesting from ambient vibration sources, which are either non periodic or have a low frequency (typically below 200 Hz). On the contrary, the enhanced flexibility of NWs enables the operation of NW generators in a quasi-static regime, so that the amount of converted energy is determined not by the operating frequency but by the number of active NWs [Ardi2012].

From ZnO NWs based piezo-generator prototypes, self-powered nano-systems without external battery have been demonstrated including pH, UV [Xu2010], chemical [Li2010] and biological sensors [Lee2011], commercial or nanostructured light-emitting diodes (LED) [Zhu2010, Chen2012b], liquid-crystal-display (CCD) screen [Hu2011] or wireless radio frequency data transmitter [Hu2011b]. Finally, piezo-generators based on ZnO NWs have been applied to selfpowered nanodevices for in vivo biomedical applications by harvesting biochemical energy [Pan2010] or a combination of biomechanical and biochemical energy [Pan2011;Hans2010;Lee2014].

Although ZnO NWs remain today the dominant material investigated for the development of NW piezogenerators, the field has been extended to other materials such as PVDF [Chan2010] or PZT nanofibers [Chen2010]. Concerning III-nitride nanostructures, to the best of our knowledge, only few macroscopic piezo-generators based on nitride NWs have been reported in the literature. The first prototype consists of multiple layers of dispersed NWs embedded in polymethyl methacrylate (PMMA) [Lin2011], while the second one is based on vertical aligned InN NWs [Liu2016], both generators exhibiting respectively an output power density of ∼ 0.3 mW/cm 3 and ∼ 0.15 mW/cm 3 (calculated from the published data). Finally, the third piezogenerator integrating a vertical array of GaN NWs was reported to generate a power density of the order of 12.7 mW/cm 3 , which approaches the best values reported for ZnO NW based piezogenerators [Zhu2012]. Piezoelectric properties of nitride NWs have also been used for other applications than energy harvesting. For example, high sensitivity pressure sensors based on GaN NWs have been reported [Zhou2013;Salo14]. In general, the interplay between the semiconducting and piezoelectric properties in nanostructures receives today an increasing attention since it offers new interesting functionalities [Wen2015].

It should be noted that there is no consensus in the literature on how to evaluate the generated power of a NW-based generator in a normative way. The electrical performance is either given in absolute values or is normalized with respect to the generator volume or surface area. Different electrical load circuits are used, so that comparison between different reports is not straightforward. In this review, when the generated power is not given, we try to estimate it from the parameters provided in the publications where it is possible.

Numerous realizations show the capability of NW piezogenerators to produce electrical power in µW -mW range. Their use for energy harvesting thus appears as a very promising concept for realizing self-powered microelectronics. However, the optimization of the mechanical-electrical conversion of individual nanowires and of the macroscopic generator architecture are crucial issues for the development of efficient devices. In this review, we address the status of these two technological challenges restricting ourselves to the specific case of III-nitride nanowires. After a short description of the main properties of III-nitride nanomaterials for the piezo-generation in section 1, the piezoelectric responses of III-N NWs and the analyses of the piezo-generation mechanism are presented in section 2. Section 3 is dedicated to the overview of the existing piezo-generators based on nitride NW arrays.

1-III-Nitride NWs.

III-nitride nanowires have been intensively studied from the early years 2000. The strong interest to these nanomaterials has been mainly stimulated by their numerous optoelectronic applications, in particular for light emission. Nitride nanowires can be elaborated by different growth techniques, the most widespread ones are Chemical Vapor Deposition (CVD), Molecular Beam Epitaxy (MBE) and Metal Organic Vapor Phase Epitaxy (MOVPE). III-N nanowires grown by MBE are particularly interesting for piezogeneration since this technique allows for an ultimate control over the nanowire properties such as their size, aspect ratio, doping level, etc. In addition, NWs with diameters as small as 20 nm can be achieved as illustrated in Fig 1. III-nitride nanowires crystallize in wurtzite structure, where the cations and anions form a tetrahedral coordination. The III-N materials are characterized by the absence of inversion symmetry, which confers them two important characteristics. The first one is the presence of polar surfaces with the metal-terminated (0001) and the N-terminated (000-1) orientation (Fig. 2). These two faces are not equivalent and present different properties, such as different surface morphologies, optical and kinetic properties [Feen2002, Huan2002, Jang2002]. The second characteristic is the existence of an internal polarization along the z-axis. This one consists of the sum of the spontaneous polarization (P SP ) and of the piezoelectric polarization (P PZ ). When the hexagonal structure is at equilibrium, i.e. not subjected to an external deformation, the centroids of positive and negative charges do not coincide. This results in the formation of a set of dipoles oriented along the same direction, leading to the appearance of a macroscopic polarization in the material, so-called spontaneous polarization. P SP is aligned with the z axis and oriented from N to Ga atoms. Thus P SP is parallel to the (000-1) growth direction and anti-parallel to the (0001) growth direction as illustrated on figure 2. The piezoelectric polarization results from the relative displacement of the Ga cations with respect to the N anions along the deformation direction when a stress is applied to the material. Its orientation along the z axis is driven both by the P PS orientation and by the strain created inside the materials. Hence, under axial compression, P PZ is oriented anti-parallel to the P PS , while it is parallel to the P PS , under axial stretching.

2-Piezoelectric properties of III-N NWs.

The performance of a piezoelectric energy harvesting system primarily depends on the piezoelectric properties of the material used to fabricate the generators. For this reason, it is of great importance to quantify the mechanical-electrical conversion efficiency and the achievable output power of NWs as a function of their structural and electric characteristics.

One of the most common methods to measure the piezoelectric effect in NWs consists in a lateral bending of the nanostructure while simultaneously measuring the generated electric potential resulting from the NW deformation. The atomic force microscope (AFM) equipped with an electrical module is a powerful tool for measuring piezoelectric properties of 1D-nanostructures. In fact, this nondestructive characterization technique combines the scanning and deflection measurements capabilities with a nanometer scale resolution and the real-time electrical measurements in usual AFM scanning conditions. The general principle of measurements is as follows: The conductive AFM tip is brought into contact with the surface of the GaN NWs under a controlled and constant normal force. During the imaging, the tip induces a local bending of the nanowires. In response to this deflection and due to the piezoelectric effect, the NWs generate a voltage which is detected through the conductive AFM tip. Throughout measurements, both the topographic and the electrical signals (output current or output voltage) are recorded continuously and simultaneously. In this specific instrumental configuration, the external electrical system is connected via an Ohmic contact formed between the NWs and the substrate, and a Schottky contact formed between the NW top and the AFM tip, which ensures the charge recuperation for piezo-generation [Liu2008]. No external voltage is applied and the voltage waveform generated by the NW is observed across a load resistance R L . Two different configurations are usually used to perform these measurements: The first one is the scanning configuration (Fig. 3a). In this configuration, the AFM probe scans laterally a vertical array of NWs. The NWs are subjected to an external lateral force resulting from the convolution of the constant normal force and the lateral force resulting from the tip/sample relative displacement. Nanostructures are bended under this external force and an electric field is created inside the nanostructure through the piezoelectric effect. Depending on the distribution of potential within the bended nanowire and the position of the conductive tip on its surface, the Schottky junction formed between the metal tip and the semiconductor NW can be reverselybiased or forward-biased. In this configuration, by soliciting successively each NW over the entire surface scanned by the AFM tip, it is possible to map simultaneously topography and voltage / current peaks. The second configuration is the perpendicular bending configuration (Fig. 3b). In this specific instrumental configuration, the sample is vertically mounted so that the conductive AFM tip applies a local force perpendicular to the free-standing NW at a selected forcing location. This method allows a direct bending of individual NWs and provides a more precise control of the applied force and its point of application. However, because the conductive AFM tip is only in contact with the stretched side of the bended NW, the Schottky diode has to be forward-biased, which implies that for this technique to be effective, a given distribution of the piezoelectric potential within the nanostructure is needed (which in turn depends on the specific characteristics of the nanostructure such as doping and polarity as detailed in section 2.2). Wang and co-workers [Wang2010b] have evidenced the detection of sharp output voltages for all the nitride nanostructures, except for AlN nanocones. Hence, outputs of about 4, 7 and 60 mV for respectively AlGaN, GaN and InN nanocones have been measured using AFM technique. The authors attributed this dispersion in the observed output voltages for the different analyzed nanomaterials to the difference in their conductivity, mainly related to the carrier density.

Following their consideration, the AlN nanostructures produced no output signal due to their very high resistivity, while the InN nanocones, due to their high conductivity, generated the largest output voltage. The promise of GaN and InN nanostructures for piezogeneration has been confirmed with the measurement of output voltages reaching 300 mV [Chen2012] and 1 V

[Huan2010] per single GaN and InN NWs, respectively (Fig. 5). This output voltage achieved for InN remains until now the highest value for any type of piezoelectric semiconductor nanowire. To better understand the piezoelectric response of III-nitride nanostructures, the group of L. Montès has investigated the output voltage generated by intrinsic GaN and GaN/AlN/GaN heterostructured NWs grown by plasma-assisted molecular beam epitaxy [Xu2011]. AFM technique in perpendicular bending configuration was employed. The authors have observed that for both types of NWs, the electrical potential increases as a function of the applied force, as illustrated in figure 6a. The impact of the rate with which the NW is deformed has also been investigated (Fig. 6b), highlighting a good stability and a high reproducibility of the piezogenerated potential. The perpendicular bending configuration of the AFM technique offers a very good control over the force and the position where this force is applied to the nanostructure. By comparing the two sets of samples under the same experimental conditions (F Applied = 250 nN), the authors observed that the intrinsic GaN NWs generated a potential of the order of 17 mV, while this potential reached 200 mV for heterostructured GaN/AlN NWs (Fig. 7a). In addition, to evidence the impact of the NW structure, the authors experimentally analyzed the relation between the NW deflection and the measured electrical generation (Fig. 7b). The deflection-generation curve can be described as composed of three parts. In the cut-off region, i.e. at the beginning of the manipulation, although the deflection is already non zero, the created piezoelectric potential is too small to allow charges to cross the Schottky barrier, and thus no output signal is detected. In the quasi-linear region, the NW deflection induces a sufficient potential to open the Schottky barrier. Output voltages can thus be measured in this regime and are found to increase linearly with the NW deflection. Finally, in the saturation region, the NW deflection becomes very large.

The internal piezoelectric potential cannot increase further due to the saturation of the rotation of electric dipoles [Dona2007], and thus the electrical output saturates.

Fig. 7: (a) Output voltage generation as a function of the NW deflection for an intrinsic GaN NW (red) and a GaN/AlN NW (blue). (b) Relation between the NW deflection and the measured electrical generation for an intrinsic GaN NW (red) and a GaN/AlN NW (blue). (Reproduced from [Xu2011].)

Piezoelectric conversion mechanisms of III-nitride NWs were further analyzed by N. Gogneau et al. [Gogn2014]. The authors investigated the output voltage generation of a vertical array of intrinsic n-doped GaN NWs as a function of the applied force by using AFM equipped with a Resiscope module in scanning configuration. With this technique, the authors acquired a large measurement statistics, with the output signal distribution reflecting the dimension dispersion of the NWs. However, because of the convolution of the normal force and the lateral force due to the scanning movement of the tip (as illustrated in Figure 3a), the nanostructures are subjected to a higher degree of deformation than when they are stressed by a normal force (perpendicular bending configuration of Figure 3b)). In these conditions, the quantitative analysis of the piezogeneration efficiency of the NWs is technically very complex due to a very strong NW deformation bringing them directly into saturation regime. In order to reduce the NW bending and thus to achieve more reliable electrical measurements, the NWs were mechanically consolidated by embedding their base part into a PMMA layer (more details can be found in Ref. [Gogn2014]). Figure 8 The increase of the force applied by the conductive AFM tip leads to a larger surface of the Schottky contact and thus to the improvement of the metal-semiconductor contact stability [Pere2007]. This improved contact allowed then a better harvesting efficiency. Several other groups have presented AFM characterizations of single piezoelectric properties of nitride NWs, then demonstrating their promise as nanogenerators [Gogn2014; Huan2010; Xu2011; Wang2010b]. On the basis of these AFM characterizations performed on free-standing nitride NWs, power densities reaching up to several hundreds of mW/cm 2 [Su2007; Xu2011; Gogn2014; Chen2012b] were estimated. These estimations are usually done for an ideal case under a hypothesis that all NWs show the same piezoconversion (by taking the highest measured output and multiplying by the nanowire density). Although this way of calculating the output power surely leads to its overestimation, it still demonstrates the potential of GaN NWs to generate high output energy. Therefore, III-N NWs can be used as an active material in ultracompact and integratable renewable energy sources for sustainable, independent and maintenance-free operation of micro-devices.

Mechanical-electrical conversion mechanisms in III-N NWs.

The enhancement of the mechanical to electrical energy conversion necessary for developing optimized nanowire based piezo-generators requires fundamental understanding of piezoelectric mechanisms taking place inside the nanostructures. Again, the AFM in scanning mode is well suited to analyze the mechanical-electrical conversion mechanisms in individual nanowires. Since this nanometer scale resolved technique allows simultaneous recording of topographic and electrical signals, it is possible to directly correlate the morphology of a given bended NW with its generated voltage peak. This correspondence was established in [Su2007, Gogn2014b] highlighting that the piezo-generation of GaN nanostructures is always observed when the AFM tip gets in contact with the stretched side of the n-doped GaN nanostructures, i.e. when the tip gets in contact with the NW and the deflection starts (Fig. 9). The opposite behaviour was observed for n-doped ZnO NWs: in this case, the output signal is generated when the tip is in contact with the compressed side of the NW [Lin2009, Wang2008].

To understand this difference, the piezoelectric properties of the nanomaterials and their doping and polarity characteristics have to be considered. In both cases, the NWs are n-type doped. This electrical characteristic is confirmed by the negative output signals measured by AFM [Lin2009].

The GaN nanostructures are also characterized by being terminated by N-polar surface (Npolarity) as it has been demonstrated by KoH etching [Su2007, Larg2012]. These two specificities are of great importance, since they determine the behaviour of the Schottky diode formed at the interface between the bended GaN NW and the conductive electrode (AFM tip).

When the GaN nanostructure is laterally bended by the AFM tip in scanning configuration, an asymmetric strain is created across the NW. The inner side of the 1D-object is compressed leading to a negative strain, while the outer side is stretched leading to a positive strain, as illustrated in Fig. 10a). As a consequence of this strain field, a piezoelectric polarization (P PZ ) appears inside the nanostructure. Under compression, P PZ is oriented anti-parallel to the spontaneous polarization (oriented parallel to the growth direction since the nanostructure are Npolar terminated) and thus oriented opposite to the growth direction, while under stretching, P PZ is parallel to the P PS and aligned with the growth direction. Due to the specific orientation of the piezoelectric polarization in the N-polar GaN NWs, the piezoelectric potential distribution in the wire evolves between V S + at the compressed facet and V S -at the stretched facet (Fig. 10b). From this potential distribution through the NW volume, two different situations occur at the Schottky contact (Fig. 10c). When the AFM tip starts to deflect the n-doped GaN nanostructure, the contact between the tip and the stretched side is a positively biased Schottky diode (ΔV = V m -V S -> 0, where V m is the potential of the metal tip, close to zero in standard conditions). Since the majority carriers are electrons, charges flow across the interface driven by the piezoelectric potential, resulting in an external output voltage detected across the load resistance. By contrast, when the conductive tip reaches the compressed side of the nanostructure, the Schottky diode is reversely biased (ΔV = V m -V S + < 0) and the charge flow across the interface is blocked, explaining the absence of output voltage. By comparison, the n-type doped ZnO nanowires are characterized by metal polarity, i.e. the NWs are terminated by Zn atoms [Wang2003, Wang2007b]. This metal-polarity of the ZnO NWs orients the P SP anti-parallel to the growth direction. The piezoelectric polarization in metal-polar structures is thus oriented in the opposite direction in comparison with the anion-polarity structures (N-terminated GaN). Consequently, the electric potential through the metal-polar ZnO NWs evolves between V S -at the compressed side and V S + at the stretched side. This difference in crystal polarity inducing a difference in potential distribution explains the different generation behaviour between n-doped ZnO and n-doped GaN nanowires. By considering this mechanism, the piezogeneration of p-doped GaN NWs should be detected on the compressed side of the NWs. In order to verify this point, Gogneau et al. have grown and characterized by AFM in scanning mode and array of N-polar Mg-doped GaN NWs [Jamo-sub]. Indeed, the generation as observed only when the AFM tip was in contacted with the compressed side of the NW due to the opposite behaviour of the Schottky diode and the sign of the output voltages was changed to positive as expected for a p-doped structures. This is illustrated in Figure 11. The results described in this section demonstrate that the piezo-generation mechanism crucially depends on the polarity and doping characteristics of the nanostructures, which is of crucial importance to design optimized piezoelectric generators and maximize their conversion efficiency.

3-Piezoelectric generators based on III-N NWs.

Investigations of the piezoelectric energy harvesting by III-nitride individual nanostructures have prepared the development of macroscopic NW-based piezoelectric generators. Today, in spite of the numerous and very promising results obtained on single nanostructures, to the best of our knowledge, only three macroscopic (i.e. with hundreds of micrometres to centimetre dimensions) piezo-generators integrating arrays of III-nitride NWs have been reported in the literature.

The first prototype has been proposed by Prof. Wang and co-workers in 2011 [Lin2011]. It consists of multiple layers of dispersed GaN NWs embedded in polymethyl methacrylate (PMMA), and sandwiched by two metal electrodes of Cr/Au, the ensemble of device being supported by a film of Kapton, as illustrated in figure 12a. To mechanically solicit this generator, the device has been attached to a flexible polystyrene substrate, to which an external force has been applied to bend the generator. Output voltages and output currents up to 1.2 V and 0.16 µA/cm 2 have been measured (Fig. 12b). By considering the dimensions of the devices, namely an effective size of 5 mm x 5 mm for a thickness of approximatively 6.4 µm, a power density of 0.16 mW/cm 3 can be estimated. (Reproduced from [Lin2011].)

The second demonstrator published very recently [Liu2016] integrates either vertically aligned pdoped InN NWs or intrinsic NWs. The nanostructures are embedded into PMMA and the Schottky contact necessary to detect piezoelectric signal is formed between the InN NWs and the MoO 3 /Au electrode (Fig. 13a). The piezo-generator devices have been tested under external dynamic strain by anchoring the substrate on a fixed stopper, leaving the top electrode facing outward. By moving up and down the system, a periodic strain was applied. The response of the device to the mechanical solicitation appears under the form of voltage (current) spikes as shown in figure 13b. Open-circuit voltage and short circuit current of ∼ 55 mV and ∼ 211 nA for p-type piezo-generator and of ∼ 85 mV and 80 nA for intrinsic-type piezo-generator have been measured for the same experimental solicitations. The power production of p-type piezo-generator has been found as 70% more than the power production of the intrinsic generator, reflecting better piezoelectric energy conversion efficiency for p-type doped NWs. embedded into spin-on glass with a Schottky contact for rectification and collection of piezogenerated carriers (Fig. 14a). For p-doped GaN NWs having nitrogen polarity, the top Schottky contact collects carriers under compressive strain. In order to homogeneously strain the active pin-on glass / NW layer, the generator is bended by clamping its two sides and inducing a vertical movement of its central part (Fig. 14b). Due to the difference of thickness between the substrate (350 µm) and the active layer (∼ 1 µm), the lateral bending of the device induces stretching of the spin-on glass matrix and, as a consequence of the volume conservation, a thinning of the active layer. The spin-on glass deformation is transmitted to the embedded GaN NWs leading to their homogeneous compression (Fig. 14b). In response to a periodic stressing, the device generates a voltage signal synchronized with the magnitude of the applied force (Fig. 14c). The piezogenerator exhibits a saturated maximal output voltage of about 200 mV, resulting in an output power density of about 12.7 mW/cm 3 . This value settles the new state of the art for piezogenerators integrating arrays of GaN NWs and more generally nitride NWs, and offers promising prospects for the use of GaN NWs for high-efficiency ultra-compact energy harvesters.

Conclusion and Perspectives

Piezogenerators based on nitride NWs show promising performance with generated power densities that are already interesting for real world applications (e.g. supplying of remote wireless transceivers etc.). However, the performance of macroscopic devices is still far from the estimations done by extrapolating the best value measured for single nanowires to the nanowire array. It is therefore clear that the optimization of the nanowire properties and in particular a better wire-to-wire homogeneity would allow to enhance the generator performance. NW arrays grown by selective area epitaxy can respond to this need. Impact of different NW characteristics (alloy composition, structure, dimensions, density and conductivity) on the piezoelectric conversion needs also to be explored.

Semiconductor NWs open the possibility for new designs and new functionalities beyond the simple size reduction. Axial or radial heterostructures can be incorporated into NWs can be used to engineer strain in the active region to further enhance the energy generation. Indeed, when a NW heterostructure contains materials with different lattice constants with layer thicknesses below the critical thickness for relaxation, an elastic strain is created inside the NW modifying the built-in electric fields and the piezoresponse. One example of the usefulness of this strategy is given by the analogy that can be drawn with existing piezo-actuators using prestrained materials to achieve symmetrical voltage response and to improve linearity. Besides, barriers of a higher bandgap material [Xu2011] can be used to localize the free carriers and thus to engineer the internal impedance of the NWs.

Along these lines, it is very likely that future research and development of NW-based piezogenerators will bring these devices to maturity and to the world of real widespread applications as miniature portable energy sources.

Fig. 1 :

 1 Fig. 1: SEM image of GaN NWs grown by MBE. Inset displays a high magnification image showing NWswith a diameter as low as 20 nm.

Fig. 2 :

 2 Fig. 2: Representation of the GaN structure for Ga-face (a) and N-face (b). The orientation of the spontaneous polarization is given as a function of the structure polarity.

Fig. 3 :

 3 Fig. 3: Schematic representation of atomic force microscope equipped with electrical module for piezoelectric generation measurement for (a) scanning configuration and (b) perpendicular bending configuration.

Fig. 4 :

 4 Fig. 4: Scanning Electron Microscope images, Generated output signal mapping and Topographicelectrical profiles of (a) GaN nanorods (reproduced from [Su2007]); (b) AlN nanocones, (c) AlGaN nanocones, (d) GaN nanorods and (e) InN nanocones (reproduced from [Wang2010b]).

Fig. 5 :

 5 Fig. 5: Scanning Electron Microscope images and generated output signal mapping for (a) GaN and (b) InN NWs, respectively [Reproduced from[Chen2012] and[Huan2010]].

Fig. 6 :

 6 Fig. 6: (a) Schematic illustration of the studied GaN intrinsic nanowires and of GaN/AlN/GaN heterojunction nanowires. Temporal trace of the generated potentials together with the applied force temporal trace. (b) Output signals generated by the NWs for different increase/decrease rate of the applied force. (Reproduced from [Xu2011].)

  presents the 3D output voltage map of a partially encapsulated GaN NW array recorded at a constant normal tip force. As Xu et al. observed on single nanowires in a quasi-linear region [Xu2011], Gogneau et al. showed a statistical enhancement (i.e. enhancement of the maximal value and of the average value) of the signal generated by the ensemble of GaN NWs with increasing the applied force. Two phenomena were put forward to explain this behavior. The first one followed the consideration of Xu et al. [Xu2011] on the potential versus NW deflection relation. As the NW deflection increases the resulting internal piezoelectric potential increases, in agreement with theoretical predictions [Gao2007b]. The second phenomenon put forward by Gogneau et al. considered the harvesting of the generated energy.

Fig. 8 :

 8 Fig. 8: 3D output voltage map of a vertical array of GaN NWs for different constant normal forces: (a) F = 4 nN, (b) F = 63 nN, (c) F = 100 nN, (d) F = 136 nN and (e) F = 173 nN. (f) Histogram distribution of the output voltages generated by GaN NWs for F = 173 nN. (Reproduced from [Gogn2014].)

Fig. 9 :

 9 Fig. 9: (a) Topographic and electrical images of GaN NWs and the corresponding topographic and electrical profiles [Gogn2014b]. (b) Topographic profile and the output current for GaN nanorods [Su2007].

Fig. 10 :

 10 Fig. 10: (a) Strain distribution in a NW bent by conductive AFM tip; (b) Potential distribution in the NW; (c) Biasing condition of the Schottky contact between the NW and the AFM tip for the stretched and compressed sides. The piezopotential distribution illustrated in panel (b) leads to a directly and a reversely biased diode, respectively. (Reproduced from [Gogn2014b].)

Fig. 11 :

 11 Fig. 11: 3D map of the output voltage generated by p-doped GaN NWs and the correlation betweenthe topographic and electrical signal profiles.

Fig. 12 :

 12 Fig. 12: (a) Schematic illustration of the GaN NW-based piezo-generator.(b) Output voltage (left) and output current (right) generated by the GaN-based piezo-generator with forward connection. (Reproduced from [Lin2011].)

Fig. 13 :

 13 Fig. 13: (a) Schematic diagram of the InN NW-based piezo-generator. (b) Measured open-circuit voltage and short-circuit current of p-type and intrinsic piezo-generators. (Reproduced from [Liu2016].)

Fig. 14 :

 14 Fig. 14: (a) Optical image and schematization of the piezo-generator fabrication process. (b) Setup used to characterize the piezo-generator. The lateral bending of the generator induces a thinning and thus a compression of the active layer as illustrated. (c) Output voltages generated by the piezogenerator as function of its deformation for a bending cycle rate of 3 Hz (top) and 6 Hz (down).

Table 1 .

 1 Piezo coefficient d 33 (pm/V) for low-dimensional and bulk GaN and ZnO.

	Material	Low-dimensional	Bulk
	ZnO	14.3-26.7 [Zhao2004]	9.93 [Zhao2004]
	GaN	12.4 [Mina2012]	2.8-3.7 [Guy1999]

Acknowledgments

This work was financially supported by the by EU Horizon 2020 ERC project "NanoHarvest" (Grant 639052), the French National Research Agency though the GANEX program (ANR-11-LABX-0014) and the Region Ile-de-France in the framework of DIM Nano-K.


Bibiana Metelmann
email: bibiana.metelmann@uni-greifswald.de

Camilla Metelmann

Konrad Meissner

Michael Wendt

Joao Goncalves

Peadar Gilligan

Ahjoku Amadi- Obi

Donal Morris

Eleni Patouni

Martin Von Der Heyden

The Potential of Telemedicine

Keywords: Telemedicine, emergency medicine, head mounted video-camera, simulation, LiveCity

Telemedicine as a communication technology to overcome geographical distances can increase the quality of medicine. The prerequisite for telemedicine is that two persons or groups of persons are connected with each other. Often this connection is built beforehand during a face-to-face-meeting when both partners apportion the communication devices. If an emergency patient is not already part of a telemedicine project, the connection has to be newly created and the device to build the connection has to be brought to the patient. In the EU-funded LiveCity-Project the hypothesis was evaluated, that in emergency situations a telemedicine connection between a patient and a remote medical doctor can be accomplished by a device brought to the patient by paramedics. It was to be established if communication with a head mounted video-camera coupled with a LTE internet connection was feasible for this purpose.

Introduction

Telemedicine represents communication and information technologies that, when together, can help diagnose and treat diseases of patients over a geographical distance [1,[START_REF]Telemedicine -Opportunities and developments in Member States: report on the second global survey on eHealth 2009[END_REF]. The word "telemedicine", which is a hybrid word containing the Greek "tele" and Latin "medicus", was first used by Kenneth Thomas Bird in the 1970s and is often translated as "medicine at a distance" [START_REF] Strehle | One hundred years of telemedicine: does this new technology have a place in paediatrics?[END_REF][START_REF] Benger | A review of telemedicine in accident and emergency: the story so far[END_REF]. Albeit there are more than 100 different peer-reviewed definitions of the word "telemedicine", there is no single agreed definition. [START_REF] Sood | What is telemedicine? A collection of 104 peer-reviewed perspectives and theoretical underpinnings[END_REF] Telemedicine is an important future topic, which is for example highlighted on a national scale in the coalition agreement of the new German Government [START_REF]Koalitionsvertrag zwischen CDU, CSU und SPD[END_REF] and on a global scope by the World Health Organization, which established a Global Observatory for eHealth [START_REF]Telemedicine -Opportunities and developments in Member States: report on the second global survey on eHealth 2009[END_REF]. There are two dimensions of communication in telemedicine. One is a horizontal communication between two or more medical experts (e.g. medical doctors in different hospitals) and the other is a vertical communication between a medical expert and a patient (e.g. medical doctor and patient at home). The vertical dimension is also between two healthcare-providers with different levels of medical expertise (e.g. emergency doctor and paramedic). Based on that, there is the possibility to have a medical supervisor for both medical staff and medical non-experts. There are several situations in which such help from a distance might be useful, e.g. a nurse in a family practice helping a patient at home with his daily treatment of chronic diseases. One goal of telemedicine is to reduce hospital admissions and to provide care to a patient in his familiar surroundings. [START_REF] Koch | Regional Health Care (RHC)[END_REF] Another goal is to treat urgent worsening of chronic diseases or emergencies. [START_REF] Shah | High-intensity telemedicine-enhanced acute care for older adults: an innovative healthcare delivery model[END_REF][START_REF] Saleh | Re-admissions to hospital and patient satisfaction among patients with chronic obstructive pulmonary disease after telemedicine video consultation -a retrospective pilot study[END_REF] The German medical emergency system consists of two partners: the paramedics and the emergency doctor [START_REF] Harding | Schlaganfall immer mit Notarzt?"[END_REF]. While the paramedics respond to every emergencycall, the emergency doctor is only alerted in certain situations. Criteria to alert an emergency doctor are severe damage of the body or danger to life. [START_REF] Harding | Schlaganfall immer mit Notarzt?"[END_REF] The evaluation of severity of every emergency case is done based on the information established during the emergency call and according to determined regulations. [START_REF] Schilling | Zuweisungskonzept bei akutem Schlaganfall[END_REF] The paramedics and the emergency doctor are then alerted independently. In 49,5% of all emergency cases an emergency doctor is alerted, too. [START_REF] Schmiedel | Leistungen des Rettungsdienstes 2008/09[END_REF] In the last decade there was an increased need for an emergency doctor detectable. [START_REF] Schmiedel | Leistungen des Rettungsdienstes 2008/09[END_REF] In 99,1% the paramedics and emergency doctors meet via the rendezvous system. [START_REF] Schmiedel | Leistungen des Rettungsdienstes 2008/09[END_REF] This means that the paramedics and the emergency doctor reach the emergency site with different vehicles independently to start working together at the emergency site [START_REF] Schmiedel | Leistungen des Rettungsdienstes 2008/09[END_REF][START_REF] Wahlperiode | Bericht über Maßnahmen auf dem Gebiet der Unfallverhütung im Straßenverkehr 2008 und 2009[END_REF]. The concept of the rendezvous system was established, because there are more paramedics than emergency doctors. Therefore it is possible to maintain additional ambulance stations with paramedics. [START_REF] Schmiedel | Leistungen des Rettungsdienstes 2008/09[END_REF]. In average the paramedics in Germany arrive after 8.7 minutes while the emergency doctor reaches the emergency site after 12.3 minutes [START_REF] Schmiedel | Leistungen des Rettungsdienstes 2008/09[END_REF]. Thus in most cases in the German medical emergency system, the paramedics reach the patient earlier than the emergency doctor.

The prerequisite for telemedicine is that two persons or groups of persons are connected with each other by the means of a communication technology. Often this connection is built beforehand during a face-to-face-meeting when both partners apportion the communication devices. When using telemedicine in emergency medicine, there is no possibility to meet beforehand and establish a connection. If an emergency patient is not already part of a telemedicine project, the connection has to be newly created and the devices to build the connection have to be brought to the patient.

Research Context

In the EU-funded LiveCity-Project this hypothesis was evaluated, where in emergency situations a telemedicine connection between the patient and a remote medical doctor can be accomplished by the means of a device brought to the patient by paramedics. LiveCity, short term for "Live Video-to-Video Supporting Interactive City Infrastructure", studies how live video connections via LTE can improve the quality of life of European Citizens in various situations. The Greifswald subgroup studied the use of this LTE video connection in simulated medical emergency situations. As shown above in most cases the paramedics reach the patient earlier than the emergency doctor does. Therefore there is a certain period of time, in which the paramedics are without guidance of an emergency doctor. In addition there are other situations in which the paramedics at the emergency site are without an emergency doctor although one is required. As explained above the emergency dispatcher team makes an initial assessment of the situation based on the alert-call. Depending on this initial assessment, the decision is made, whether to alert an emergency doctor or not. There are some cases, in which the need for an emergency doctor is not apparent from the beginning, so that the emergency doctor isn't alerted. The LiveCity-Project examines these situations -paramedics being without the help of an emergency doctor at the emergency site. If the paramedics want to consult a medical doctor, they could do that via telemedicine. There is for instance the possibility to connect via a live-video so that the medical doctor can see the patient and his vital signs as done in the LiveCity-Project. Based on that, he is able to help with diagnostics and treatment, therefore minimizing the time it takes to start the right treatment. Starting the right treatment as early as possible is one of the main goals of emergency medicine, e.g. when treating myocardial infarction, stroke or trauma [START_REF] Members | ESC Guidelines for the management of acute myocardial infarction in patients presenting with ST-segment elevation: The Task Force on the management of ST-segment elevation acute myocardial infarction of the European Society of Cardiology (ESC)[END_REF][START_REF] Lyden | Thrombolytic Therapy for Acute Stroke -Not a Moment to Lose[END_REF][START_REF] Sharma | Principles of diagnosis and management of traumatic pneumothorax[END_REF]. In 2002, the European Resuscitation Council (ERC) established during a conference held in Florence, Italy, the term "First Hour Quintet" [START_REF] Krafft | European Emergency Data Project EMS Data-based Health Surveillance System Project Report[END_REF][START_REF] Nilsen | Improving quality of care in the Emergency Medical Communication Centres (EMCC) in Konferanse for medisinsk nødmeldetjeneste 7[END_REF][START_REF] Lackner | Von der Rettungskette zum akutmedizinischen Netzwerk[END_REF]. This concept states five life threatening diseases, in which fast medical treatment is crucial for medical outcome concerning morbidity and mortality. Examples are stroke, myocardial infarction and traffic accidents, which are among the leading causes of death in Europe [START_REF] Fischer | Factors influencing outcome after prehospital emergencies -a european perspective[END_REF][START_REF] Fischer | Emergency medical services systems and out-of-hospital cardiac arrest Cardiac Arrest[END_REF]. To improve the therapy of stroke the American Heart Association and American Stroke Association gave recommendations to implement the use of telemedicine in emergency treatment of stroke in 2009. [START_REF] Schwamm | A review of the evidence for the use of telemedicine within stroke systems of care: a scientific statement from the American Heart Association/American Stroke Association[END_REF] In the LiveCity-Project the telemedicine-connection between the patient experiencing an emergency and the emergency doctor stationed at a hospital was to be established with a video-camera with a LTE Internet connection. In this project the remote emergency doctor is called "Tele Emergency Doctor". The video-camera, which was especially developed for this purpose, was brought to the patient by the paramedics. One paramedic wears the camera with a headband, enabling the Tele Emergency Doctor to see the same things the paramedic is seeing. Therefore the paramedic was able to show to the Tele Emergency Doctor the patient, his vital signs and all other important information while having both hands free to work. Moreover the Tele Emergency Doctor was thus able to help the paramedic with diagnosing and treating the emergency. For ethical reasons the study was not done with patients but was simulated in realistic and praxis-oriented scenarios in a simulation center. The study was designed as a randomized, two-armed crossover study trial. 2 paramedics treated together as a team 5 "patients" without the Tele Emergency Doctor (control cases) and 5 "patients" with help of the Tele Emergency Doctor (experimental cases). The paramedics approached 10 different scenarios, which were especially developed for this study. Those 10 scenarios were matched pairs belonging to one of the five categories: stroke, myocardial infarction, traffic accidents, pregnancy and rare diseases. While the first three categories were chosen, because they are part of the First Hour Quintet (see above), the last two were created due to the fact that they are a special challenge for the paramedics and emergency doctors and require exceptional expertise. Every scenario was independently evaluated by 3 paramedics and 3 medical doctors regarding practicability. To minimize influences of confounders, there was a special randomization regarding the chronological order of scenarios. Each scenario was equally tested as a control case and experimental case. Every first group started with the control cases and every second group with the experimental cases. For all scenarios the mannequin called Laerdal Resusci Anne was used. In every scenario it was initially placed either on a sofa or on the floor, depending on the emergency simulated, but the paramedics were able to move it around as needed. During the study the paramedics and emergency doctors were asked to fill out specially developed questionnaires. The questions covered different topics, including medical, technical, practical and psychological aspects.

Results

10 emergency doctors and 21 paramedics aged between 18 and 50 years were recruited. More than 350 questionnaires were completed. The paramedics were asked, whether they would prefer in their daily work to have more help with practical, manual skills (e.g. endotracheal intubation) or with finding the right diagnosis and establishing the right therapy. More than two-thirds answered, they encounter more often situations, in which they need theoretical rather than practical support (Fig. 1).

Fig. 1 Kind of requested Support

Furthermore roughly two-thirds stated that they would call the Tele Emergency Doctor in cases, where they normally wouldn't call an emergency doctor (Fig. 2).

Fig. 2 I would call a Tele Emergency Doctor in cases I wouldn't call an Emergency Doctor

After finishing all the simulations, the paramedics were asked to rate the assertion "I considered the Tele Emergency Doctor as helpful." 71% answered "agree", 29% "partly agree" and no one "partly disagree" or "disagree" (Fig. 3).

Conclusions

This study analyzed the use of telemedicine with a head-mounted video camera in emergency medicine. Paramedics, who were alone at the emergency site, got assistance of a Tele Emergency Doctor, who was watching the video in another room on a laptop and could communicate with the paramedics via the camera. The paramedics stated, that they encounter more often situation in which they need theoretical help than practical help. This theoretical support can be given not only by an emergency doctor at the emergency site, but also by a Tele Emergency Doctor via a camera. As shown by Figure 1-3 a Tele Emergency Doctor would be helpful. Furthermore participants got the feeling that a Tele Emergency Doctor could decrease the time it takes to start the right treatment. As explained above, time is a very crucial point in emergency medicine. An earlier start of treatment can lead to a significant decrease in morbidity and mortality and save life.

One limitation of this study is the relatively small number of participants.

It could be shown, that a communication with this headmounted camera with LTE Internet connection is feasible. Hence it can be concluded, that a videoconnection between paramedics at the emergency site and a Tele Emergency Doctor is practical and that it could potentially be accomplished with help of LiveCity technology.

The authors of this study would further like to conclude, that a Tele Emergency Doctor might be a very good addition to the existing German medical emergency system. Further studies are on the way.

Fig. 3

 3 Fig. 3 Perception of helpfulness

Fig. 4

 4 Fig. 4 Perception of influence on start of therapy

Acknowledgments:

The present article has been structured in the context of the LiveCity ("Live Video-to-Video Supporting Interactive City Infrastructure") European Research Project and has been supported by the Commission of the European Communities -DG CONNECT (FP7-ICT-PSP, Grant Agreement No.297291).


Andreea Molnar
email: andreea.molnar@asue.edu

Vishanth Weerakkody
email: vishanth.weerakkody@brunel.ac.uk

Ahlam Almuwil
email: ahlam.almuwil@brunel.ac.uk

Promoting ICT Skills through Online Services: Case Study of Video use for Adult Education in Municipalities

Keywords: public services, adult education, video-to-video, smart cities

The usage of video communication in public services can alleviate some of the existing problems these services face in their adoption and also help in increasing the level of outreach to citizens. In this article we focus on the usage of video-to-video (V2V) communication in municipality services for ICT education of senior citizens. For this purpose, our study focuses on determining criteria that are important for both the teachers and citizens in using such an innovative service. This paper reports on the findings from focus group consultations conducted with teachers and students, in this case senior citizens, of a municipality in Spain who was piloting the use of V2V services for adult education. The research identified several criteria relevant for consideration when deploying V2V as part of a local government service portfolio, of which, trust, quality of service and convenience emerged as common for all stakeholders involved.

Introduction

With the increased digitization of society, the usage of information and communication technology (ICT) in public services has seen a rapid increase during the last few years. Although governments have continued to push digital applications to the front line of public services, not all public services are feasible to be offered online or when offered, their adoption rate has not always been as high as expected [START_REF] Cordella | Socio Technical Regimes and e-Government Deployment: The Case of Italian Judiciary[END_REF][START_REF] Norris | Local e-Government in the United States: Transformation or Incremental Change?[END_REF]. It is likely that some services have not been used due to the lack of awareness and interest on the part of citizens or those citizens were trying to use some of these online services but failed to fully complete them and ended up using other means [START_REF] Andersen | Frequency and Costs of Communication with Citizens in Local Government[END_REF]. For some other services, deep-rooted issues were found such as accessibility, lack of inclusion of the elderly and the difficulty to provide the same rich interaction as they find in face-to-face services. This is particularly true when dealing with complex services such as health and education and social or domestic services such as social security, housing or employment [START_REF] Andreassen | European Citizens' use of e-Health Services: A Study of Seven Countries[END_REF][START_REF] Santana | Online Communication between Doctors and Patients in Europe: Status and Perspectives[END_REF]. This research focuses on education for senior citizens.

The poor adoption of eGovernment has led to public services being available both online and also mirrored via other channels [START_REF] Andersen | Frequency and Costs of Communication with Citizens in Local Government[END_REF]. However, it is not always possible to ensure that human resources are constantly available or that it is possible to reach people that may need a certain service, but are not located closely to the centres that offer these services. This is particularly true in the current environment, where local governments are facing growing deficits, loss in revenues and increasing costs [START_REF] Rosenbloom | Government Entrepreneurs: Incentivizing Sustainable Businesses as part of Local eEconomic Development Strategies[END_REF] while at the same time dealing with citizens whose usage of ICT in the private sector has led to creating new expectations [START_REF] Andersen | Frequency and Costs of Communication with Citizens in Local Government[END_REF], of high quality from services available in the public sector [START_REF] Hazlett | E-Government: The Realities of Using IT to Transform the Public Sector[END_REF].

A potential solution to these problems is the usage of high definition (HD) video technology in the provisioning of public services [START_REF] Weerakkody | Complementing e-Government Services Through the Use of Video: The LiveCity Project[END_REF]. This can imitate the richness found in face-to-face communication while preserving the efficiency, speed and outreach of e-government services [START_REF] Weerakkody | Complementing e-Government Services Through the Use of Video: The LiveCity Project[END_REF]. Moreover, the involvement of citizens from the beginning of a service design and diffusion and finding out what their needs are could increase the likelihood for a service to be adopted [START_REF] Axelsson | Talking to, not about, Citizens-Experiences of Focus Groups in Public e-Service Development[END_REF]. This research seeks to explore criteria for the adoption and the usage of HD V2V communication in public services, focusing on continuous education for senior citizens. To do so, we performed focus groups with the municipality staff, the senior citizens using these services and volunteers that were helping for these services to be delivered, in Spain. This empirical research was performed in the context of the LiveCity project which is funded by the European Commission's ICP programme (EU FP7 CIP grant agreement No. 297291). The remaining sections of the paper are structured as follows. The next section presents the background of technology usage in public services and focuses on V2V communication in particular. The next section briefly introduces the proposed service and how V2V will be ensured. The following section presents the study performed. We continue with conclusions and the discussion, and the paper ends with future work.

Background

The increasing availability of the Internet and the development of ICT have made electronic services an attractive solution for delivering public services. Several benefits of using ICT in public services are cited in the literature as factors driving adoption: increasing efficiency and decreasing the cost of providing services [START_REF] Affisco | E-Government: A Strategic Operations Management Framework for Service Delivery[END_REF], availability anytime and anywhere [START_REF] Bekkers | The Myths of e-Government : Looking Beyond the Assumptions of a new and better Government[END_REF], being able to provide more personalised solutions that put citizens in the center [START_REF] Dunleavy | New Public Management is Dead -Long Live Digital Era Governance[END_REF], and providing more transparent and accountable services [START_REF] Dwivedi | Moving towards Maturity: Challenges to Successful e-Government Implementation and Diffusion[END_REF]. It is considered that an e-government service is successful when that service is highly adopted. The usage of HD V2V has been seen as a solution for providing personalised services and as a means to offer help for citizens that have problems using current egovernment services [START_REF] Weerakkody | Complementing e-Government Services Through the Use of Video: The LiveCity Project[END_REF]. Several preliminary studies have been performed on determining criteria for the usage of V2V in education [START_REF] Molnar | Facilitating the Adoption of Public Services Using High Definition Video: The Case of Primary Education[END_REF][START_REF] Weerakkody | Indicators for Measuring the Success of Video Usage in Public Services: The Case of Education[END_REF] but to the best of our knowledge they have not yet focused on local e-government services such as continuous education for senior citizens. In addition, although prior research has identified some common criteria for the usage of HD video, it is necessary to define these criteria based on the context in which the service is used [START_REF] Dabholkar | A Measure of Service Quality for Retail stores: Scale Development and Validation[END_REF] as prior research has shown failed attempts to measure new contexts when using general scales [START_REF] Dagger | A Hierarchical model of Health Service Quality Scale Development and Investigation of an Integrated Model[END_REF].

Education increases the well-being of an individual and strengthens their independence [START_REF] Bronswijk | Technology for Quality of Life: An Enriched Taxonomy[END_REF] and for senior citizens, it makes them live more productively and less isolated [START_REF] Van Dyk | Mobile Phones for the Elderly: a design framework[END_REF]. When lifelong education is offered to senior citizens, it is mostly focused on the usage of information technology [START_REF] Zumarova | Senior Citizens as a Specific non-Traditional Group of Students in a Lifelong Education[END_REF][START_REF] Dianti | What's Up: Fostering Intergenerational Social Interactions[END_REF]. To do so, these communities need to have some basic ICT skills. Being able to access a program that teaches basic ICT skills is significantly influenced by the access to training by senior citizens [START_REF] Zumarova | Senior Citizens as a Specific non-Traditional Group of Students in a Lifelong Education[END_REF]. It has been shown that having computer skills can positively affect the life of citizens and improve their social relationships [START_REF] Zumarova | The role of ICT in the Lives of Senior Citizens[END_REF]. However the resources allocated to teach senior citizens basic information technology skills are usually limited [START_REF] Zumarova | Senior Citizens as a Specific non-Traditional Group of Students in a Lifelong Education[END_REF]. Therefore it is necessary to find new ways to give access for more citizens to these types of services. To the best of our knowledge the study covered in this paper is the first study that attempts to use V2V technology to teach senior citizens computer skills.

Research Context

This research is done in the context of the LiveCity project. It seeks to provide HD video over public infrastructure [START_REF] Weerakkody | Complementing e-Government Services Through the Use of Video: The LiveCity Project[END_REF]. LiveCity is user community driven and is aimed at improving core public services such as education, public administration, health and city experience in different European cities. The way LiveCity aims to ensure HD video delivery has already been discussed in our previous studies [START_REF] Weerakkody | Complementing e-Government Services Through the Use of Video: The LiveCity Project[END_REF] and it is out of the scope of this article to examine this again. This paper focuses on education for senior citizens. The usage of V2V for these services will happen in Valladolid, Spain. The usage of V2V for providing education for senior citizens would allow one teacher to connect to several citizens at any given time. For example, V2V will allow teaching two classes at the same time, one face-to-face and another through a video connection or having senior citizens connecting from their home. This would allow more senior citizens to benefit from lifelong learning and eliminate the need for having several teachers or facilitators, which from a municipality perspective will be one of the biggest challenges when delivering such a programme. This could also allow people that do not live in the proximity of a municipality center where these courses take place, to connect from their home. The challenge in this instance is whether the senior citizens will accept this type of technology and if they will be able to use it. This is a significant challenge as the citizens will need to make use of the ICT skills they are learning through V2V in order to receive the training. Therefore, the acceptability and the way one would design the interface to make it suitable for learners are among the challenges one faces when offering such remote training.

The service focused in this paper was originally designed by the Municipality of Valladolid as a means of helping adult citizens enhance their ICT skills. At present, volunteers teach the classes that take place in one of the venues offered by the munic-ipality using face-to-face communication. The motivations for considering video to facilitate this service has been influenced by the fact that these classes are typically overbooked and currently there are not enough volunteers to teach them on a face-toface basis. Due to the current economic climate there are no plans for the municipality to increase the number of classes or to hire teachers to compensate for the demand, therefore V2V communication is seen as an appropriate solution to increase the service offered without additional cost to the municipality. Moreover, the elderly population is increasing and the usage of ICT can allow them to live a more independent, productive and less isolated life [START_REF] Van Dyk | Mobile Phones for the Elderly: a design framework[END_REF]. Senior citizens, particularly those whose families live away from home, are often motivated to take these classes as they use ICT skills gained to communicate with their families.

Study

The aim of our study was to determine which criteria would be important for the adoption of V2V services, if they were offered to citizens as part of their municipality services. To achieve this aim, we focused on consulting the stakeholders using a qualitative approach. The motivation for adopting a qualitative, rather than a quantitative (such a survey) is because it allowed the researchers to get close to reality as possible and explore criteria that need to be considered from a user perspective for such a new type of service to be successful.

Methodology

Exploratory research was chosen due to the scarcity of empirical work related to the usage of V2V in the public sector for this particular service. Case study research is useful when dealing with broad and complex phenomenon, and the existing body of knowledge is insufficient to permit posing casual questions [START_REF] Yin | Case Study Research: Design and Methods[END_REF]. Due to the lack of understanding and the complexity involved in the area of research under investigation, focus groups were used as the primary data collection method. It was not necessary to design a structured agenda to ask questions in a specific order [START_REF] Yin | Case Study Research: Design and Methods[END_REF], but literature reviews provided the themes to be explored during the empirical work. Focus groups are useful in providing the necessary focus needed to probe a research domain that is exploratory at present [START_REF] Yin | Case Study Research: Design and Methods[END_REF]. As people have different views and perspective over the same things focus groups are a suitable method to understand user views [START_REF] Kitzinger | Introduction: the Challenge and Promise of Focus Groups[END_REF]. Moreover, focus groups, when used in an e-government context, were found to be helpful in finding citizen's needs and expectations for new services [START_REF] Axelsson | Talking to, not about, Citizens-Experiences of Focus Groups in Public e-Service Development[END_REF]. Two focus groups were organised and recorded as this allowed for an easier analysis of the information. A thematic analysis process [START_REF] Boyatzis | Transforming Qualitative Information: Thematic Analysis and Code Development[END_REF] was used for the analysis, where emerging issues were linked to the criteria identified through literature and documenting any new issues and assigning labels to these.

The participants were given an information sheet containing information about the project, what is expected from them and their rights to withdraw from the study anytime without any prior notice or explanation and/or not to respond to the questions they do not feel content in responding. Participants demographic data were collected prior to the interview through a questionnaire, however the participants did not have to disclose their demographic data if they did not want to. The focus group was facilitated by a moderator that introduced the scope of the focus group and guided the interview.

The participants in the study were recruited by the local partners in the LiveCity project. The focus groups were held in Spanish and the results translated to English with the help of the Municipality of Valladolid.

Participants

We held focus groups with the teachers and the students all senior citizens who are part of the ICT education pilot. The teachers were males and over 56 years of age. A focus group was held with the students who took part in one of the classes. A total of fifteen students participated in the focus groups, 40% were males and 60% females. They were all over 56 years of age (53% belonged to the 56-65 age group and 47% were over 65 years). Their education level varied from those who have finished primary education (47%), to those who had postgraduate university education (7%). Most of them reported having used an online (e-government) municipality service for less than one year (94%) and the rest reported using it between two to five years. The teachers were volunteers and the students (senior citizens) lived in the municipality and its suburbs. All students in the ICT class were invited to participate in the focus group, no incentives were offered and all the students participated in the study.

Results

Considering the importance of involving relevant stakeholders in the implementation of ICT in public services [START_REF] Goel | Role of Key Stakeholders in Successful e-Governance Programs: Conceptual Framework[END_REF], in this study we aim to involve the volunteer teachers and the senior students. In this respect it is important to find out what criteria is necessary for these services to be successfully adopted. We followed a qualitative approach by performing focus groups with the teachers and students. We used both an inductive and deductive approach when analysing the data. When criteria emerged it was linked with the previously found criteria from the literature. If any new criteria were found, they were added to the existing list.

Teachers. A focus group was held with teachers involved in the programme. The teachers were senior citizens who knew how to use technology, some of them being retired teachers. They mentioned as their concern problems with the technology and the fact that the citizens/students are typically beginners in using a computer and therefore distrust them, but they do not see teaching these courses as difficult or that the students have a problem in getting involved. They were keen to ensure that the V2V technology used allowed them to interact with individual students as and when they had a question or needed individual attention. The teachers highlighted the need for the V2V system to incorporate features that will allow individual students to raise their hand or alert the teacher when they needed to speak in a manner that would not be intrusive to the other participants in the class, but would allow them to listen and benefit from any questions raised by others. In addition, quality of the video and voice transmission was identified by the teachers as one of the most important criteria that they were looking for in a remote, online education programme. The teachers viewed the proposed system as different to other online and distance education programmes as it allows real time two way V2V making the programme as close to face-to-face as possible. Table 1 summarizes the main criteria identified by the teachers.

Criteria Proposed Description Trust

Students confidence in the system, believing that the system is reliable Access and Availability

The system must be available and easily accessible by the citizens/students Quality of Service

The system must allow high quality live video and voice Two way interaction

The system has to facilitate seamless two way communication between teacher and student Ease of use and presence

The system should be as close to a real class room experience for the teachers and students

Table 1. Criteria for V2V based Education Services as Identified by Teachers

Senior Citizens/Students. The focus group took place with the students of one class involved in learning computer use as part of their municipality service. A total of 15 citizens participated in the service. The students were beginners and have little or no experience in using V2V services. Trust is mentioned as one of the biggest obstacles for them in using online services as well as the fear of "doing something wrong", "breaking something". They mentioned that having the V2V connection would allow more students to be enrolled and allow other students who are on the waiting list to participate in adult education. Overall, all the students were supportive of the idea, "I like the idea". They stated that they would not mind being either in the class with the teacher or accessing the course online, as long as the teacher is interacting with them and is available "to answer questions". Some of the students suggested that video based learning will be convenient for them as they presently have to travel long distances to be physically present at the adult education class. They also thought that video based learning will be better than having to be part of a large class with others where senior citizens can easily be distracted due to the social interactions that occur in the classroom. In terms of the 'look and feel' of video based learning, the students suggested that the system should have the option where they could switch or split the screen when needed between seeing the live video of their teacher and the specific actions that need to be carried out involving the keyboard to complete an ICT function. In general, the students' needs and expectations mirrored those of the teachers as they were keen to see a system that was as close to a real classroom experience as it can be. Table 2 outlines the main criteria discussed during the focus groups.

Criteria

Proposed Description Trust Confidence in the system, believing that the system is reliable and secure Confidence Students belief that they could use the system Number of courses Number of courses available through the V2V service should exceed those currently offered using classroom based teaching Individual feel of the system

The system should allow individual interaction irrespective of the number of students that are taught through the V2V service Convenience

The video based learning should be convenient for those finding it difficult to travel to a physical class room due to distance Ease of use

The system should be easy to use and should feature live video of both the teacher and his/her computer/keyboard when needed Cost Should be cost effective for citizens when comparing internet usage against travel expenses incurred to attend a physical class

Summary

This paper presents a possible usage of video in public services, focusing on lifelong learning education for senior citizens. Focus groups were performed with volunteer teachers and senior citizens taking part in an education program offered by the municipality. The results show that all participants are positive about the introduction of V2V services in their municipality. The usage of V2V was seen as a way to allow more citizens to participate in education activities. Fig. 1 summarises the criteria important both from the teachers and students point of view. Trust in technology is common to both groups, whereas the rest of the criteria are group specific.

Discussion

Although from a supplier's perspective, the usage of ICT in public services is seen as a tool for automating and streamlining service delivery [START_REF] Grant | Developing a Generic Framework for e-Government[END_REF], in the example studied in this paper ICT is seen as a means to reach out and provide access to services to as many citizens as possible. This was highlighted in the service offered to the senior citizens in the Valladolid municipality. In the context of their adult education programme, the municipality of Valladolid is expected to teach more people at the same time, hence increasing the teacher's (and the municipality's) productivity. As the teachers are all volunteers no direct cost saving are seen for the municipality, but the education programme offered by the volunteers is seen as a value adding service to the municipality in terms of public perception. Citizens see the usage of V2V communication in a very positive light.

Considering the dual role of public services, to increase efficiency and be useful to citizens [START_REF] Axelsson | Talking to, not about, Citizens-Experiences of Focus Groups in Public e-Service Development[END_REF], this article has been focused on the later by aiming to better understand the needs of citizens when using a new service and determine criteria to be taken into account during the implementation and deployment of such services. By consulting the stakeholders (users and voluntary teachers) during the design and delivery of V2V services, we have tried to align the needs of the municipality with citizens' needs and expectations. It is hoped that this approach will result in better acceptance of online services in general [START_REF] Axelsson | Talking to, not about, Citizens-Experiences of Focus Groups in Public e-Service Development[END_REF] and V2V in particular, as a mechanism for adult education as well as other wider municipality services.

Theoretical and Practical Implications

This study adds to the state of the art by determining criteria that have to be considered for the adoption of V2V communication in the public sector as a mechanism for online service delivery. In particular, it adds to the literature of public services and adult education. To the best of our knowledge this is the first study to explore criteria necessary from the citizens' perspective when using V2V for learning ICT as part of a lifelong learning program for senior citizens. The results presented in this research can be used by policy makers and managers and municipalities providing such services to determine what criteria are seen as relevant from the citizens' perspective in order to introduce new services or to improve the already existing ones. These criteria could also be used as a means of evaluating existing services.

Limitations

The results of this study are based on potential criteria as determined from focus groups with teachers and students. None of them have used the V2V system before or were involved in a similar project. Although the teachers were ICT savvy and used similar V2V services (e.g. Skype), the students did not. This may have led to difficulties in expressing what they expect from such a system [START_REF] Flak | On the evolution of e-government: The user imperative[END_REF]. The students may have also overestimated their ability to use the system without having a teacher in the class given their low level of ICT skills. The study is performed in a single location in one country and therefore cultural differences may lead to different results if the study was to be performed in another location.

Future Work

The results of this study have been taken into account when the service was implemented and deployed. Currently a small pilot is taking place in Spain. The study performed here was used to inform the design of the services and the evaluation process.

The data about citizens and the municipality experience with the service is currently being collected and will be reported. Based on these results, improvements to the two services will be made and other criteria that may be found as a result of the participants' interaction with the system will be taken on board to continuously improve the systems. We aim for the end result of this research to be a complete taxonomy of criteria that are relevant for V2V services in the public sector. The taxonomy will be evaluated in the last stages of this research when this service will be offered on a larger scale as part of the Smart City infrastructure implemented by Valladolid in Spain.

Fig. 1 .

 1 Fig. 1. Citizens generated Criteria for using V2V Communication for Adult Education.

Table 2 .

 2 Summary of Criteria for V2V services based on Citizens/Students' Views

	5	Discussion and Conclusion

Acknowledgements . The authors acknowledge the contributions made to this article by the LiveCity consortium of partners who are funded by the European Commission, especially to National & Kapodistrian University of Athens for helping in organising the study presented in the paper.