Abstract:
Various examples are provided for pillar array photo detectors. In one example, among others, a photo detection system includes an array of substantially aligned photo sensitive nanorods extending between first and second electrodes, and a plurality of resistance monitoring circuits coupled at different positions about the circumference of the electrodes. In another example, a photo detector includes first and second electrodes, and an array of substantially aligned photo sensitive nanorods extending between the substantially parallel electrodes. Light passing through an electrode excites electrons in the photo sensitive nanorods that are illuminated by the light. In another example, a method includes illuminating a portion of a photo detector including an array of substantially aligned photo sensitive nanorods with a light spot, obtaining resistance measurements at a plurality of locations around the array, and determining a position of the light spot on the photo detector based upon the resistance measurements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of, and priority to, co-pending U.S. Provisional Application No. 61/903,057, entitled “Pillar Array Photo Detector” and filed on Nov. 12, 2013, the entire contents of which are hereby incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    A light spot position detector (LSPD) can be used for system control in various industrial, military and other daily applications. The performance of a precise control system can heavily depend on how precisely the control system can detect a feedback light spot. Presently adopted technology for sensing the spotlight position is based thin film photo detectors. Two-dimensional (2D) photo sensitive devices based on thin film techniques, such as complementary optical metal-oxide-semiconductor (CMOS) or charge-coupled device (CCD), can achieve light detection with micrometer resolutions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0004]      FIG. 1A  is a graphical representation of an example of a two-dimensional (2D) thin film photo sensitive device (PSD) in accordance with various embodiments of the present disclosure. 
           [0005]      FIG. 1B  is a graphical representation of an example of a three-dimensional (3D) pillar array device (PAD) in accordance with various embodiments of the present disclosure. 
           [0006]      FIGS. 2A and 2B  illustrate testing of zinc oxide (ZnO) nanorods used in the 3D PAD of  FIG. 1B  in accordance with various embodiments of the present disclosure. 
           [0007]      FIGS. 2C through 2E  are plots illustrating characteristics of the ZnO nanorods of  FIG. 2B  in accordance with various embodiments of the present disclosure. 
           [0008]      FIG. 3A  is a graphical representation of an example of a light spot position detector (LSPD) based upon the 3D PAD of  FIG. 1B  in accordance with various embodiments of the present disclosure. 
           [0009]      FIG. 3B  is an electrical model of a resistance measurement circuit connected to the LSPD of  FIG. 3A  in accordance with various embodiments of the present disclosure. 
           [0010]      FIGS. 3C and 3D  are plots illustrating characteristics of the LSPD of  FIG. 3A  in accordance with various embodiments of the present disclosure. 
           [0011]      FIGS. 4A and 4B  are images of a fabricated LSPD of  FIG. 3A  in accordance with various embodiments of the present disclosure. 
           [0012]      FIG. 4C  is a graphical representation of a system used for multi-cycle testing of the fabricated LSPD of  FIG. 4A  in accordance with various embodiments of the present disclosure. 
           [0013]      FIG. 4D  includes plots illustrating characteristics of the fabricated LSPD of  FIG. 4A  in accordance with various embodiments of the present disclosure. 
           [0014]      FIGS. 5A through 5C  are images of a LSPD of  FIG. 3A  that can be used with a lens in accordance with various embodiments of the present disclosure. 
           [0015]      FIG. 5D  is a plot illustrating characteristics of the LSPD of  FIGS. 5A-5C  in accordance with various embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Disclosed herein are various examples related to pillar array photo detectors. Reference will now be made in detail to the description of the embodiments as illustrated in the drawings, wherein like reference numbers indicate like parts throughout the several views. 
         [0017]    Referring to  FIG. 1A , shown is a graphical representation of an example of a two-dimensional (2D) thin film photo sensitive device (PSD)  103 . When a portion of the 2D thin film photo sensitive device  103  is illuminated by light  106 , the illumination changes the concentration of the electrons that will spread through the film. Without lateral confinement, the light excited electrons in the 2D thin film PSD  103  form a diffusion gradient  109 . As illustrated in  FIG. 1A , the diffusion gradient  109  leads to an uneven diffusion of the electrons within the film of the 2D thin film PSD  103 , which adversely affects the light position detection resolution. The most common method to increase resolution is to decrease the pixel size, which is, however, limited by the manufacturing process. More importantly, noise is a negative factor for higher resolutions since the 2D thin film PSD  103  is more vulnerable to noise with the smaller pixel size. Presently, CMOS and CCD can achieve micrometer resolution, which approaches their physical size limitation based on the thin film technique. 
         [0018]    Nanoscale photo sensitive structures can be produced using three-dimensional (3D) pillar arrays of nanorods. A 3D pillar array device (PAD) includes a plurality of vertically aligned nanorods (or nanowires) as pixels.  FIG. 1B , shows a graphical representation of an example of a 3D PAD  112  including photo sensitive nanorods. For example, vertically aligned zinc oxide (ZnO) nanorods (or nanowires), with diameters of tens of nanometers, can preserve the detectable photoelectric response when the heights are above a predefined value. In other embodiments, the nanorods may comprise other photosensitive semiconducting materials such as, e.g., GaN, Si, Al 2 S 3 , InSb, Ge, Se, PbS, Bi 2 S 3 , etc. When a portion of the 3D PAD  112  is illuminated by light  115 , the light excited electrons are confined within the boundaries of the corresponding nanorods (or nanowires) without lateral dispersion as illustrated in  FIG. 1B . This feature of the 3D PAD  112  overcomes the electron dispersion in the continuous thin film of  FIG. 1A . A large oxidization enhanced surface volume ratio (high oxygen absorption) enables the ZnO nanorods (or nanowires) to exhibit a strong photoelectric effect, resulting in nanoscale resolution. 
         [0019]    Different from the thin-film technique, which only concerns 2D size, the detectable photoelectric characteristic of ZnO nanorods is not controlled by its 2D radial size. Instead, the detectable photoelectric characteristic is determined by the nanorod&#39;s 3D volume. The photoelectric effect resistance changes with respect to the volume of ZnO. In this way, the size limitation of 2D photo sensitive pixels may be overcome by using a 3D pixel. Based on this, a LSPD using grown ZnO nanorod arrays was implemented with a measured resolution of 200 nm. In other embodiments, the resolution may be improved to 10 nm or even smaller resolutions. The pixel size of the LSPD (the diameter of aligned nanorods) may be reduced to the tens of nanometer range, while the height of the pixel compensates the volume that is needed to be over the detectable photoelectric limitation. The spacing between the nanorods of the array may be about 500 nm or less, about 200 nm or less, about 100 nm or less, about 50 nm or less, about 20 nm or less, or about 10 nm or less, comparable with the diameter of the nanorod to provide the appropriate resolution. 
         [0020]    In nanoscale, the basic properties of materials deviate from those of their bulk counterparts. The effect of size on the detactable photoelectric effect of ZnO nanorods (or nanowires) was examined through experimentation. It was found that the photoelectric effect of a ZnO nanorod becomes undetectable when its volume is small enough. Referring to  FIGS. 2A and 2B , shown are examples of testing of ZnO nanorods with different volumes. The scanning electron microscope (SEM) image  203  of  FIG. 2A  illustrates short ZnO nanorods that were grown with a radius of 25 nm and a height of 40 nm on a conductive substrate and the SEM image  206  of  FIG. 2B  illustrates tall ZnO nanorods that were grown with the same radius of 25 nm and a height of 2 μm on a conductive substrate. 
         [0021]    Using Current Atomic Force Microscopy (I-AFM, Park XE-70), the conductivity of the short and tall ZnO nanorods were directly measured under illumination and in a dark environment as graphically illustrated in  FIGS. 2A and 2B . The resulting current-voltage (IV) curves are shown in plots  209  and  212  for the short and tall ZnO nanorods, respectively. The curves  215  and  218  are the IV curve under illumination and the curves  221  and  224  are from the dark environment. In plot  209 , the currents of the short ZnO nanorod under illumination (curve  215 ) and in the dark (curve  221 ) overlap making identification difficult. In contrast, the currents of the tall ZnO nanorod under illumination (curve  218 ) and in the dark environment (curve  224 ) are clearly different. Performing these measurements on ZnO nanorods with the same radius and different heights, it was found that the photoelectric effect becomes detectable as the height increases. The insets  227  and  230  of plots  209  and  212  depict the topographies of the short and tall ZnO nanorod to illustrate physical sizes. The insets  227  and  230  were obtained by AFM working in tapping mode and the scale bars illustrate a distance of 25 nm. 
         [0022]    To explain the measured phenomenon, an analytical model was developed to describe the average electron density of ZnO nanorods in a dark environment at 300K. The dark electron density (n dark ) may be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       n 
                       dark 
                     
                     = 
                     
                       
                         ( 
                         
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                               rod 
                             
                           
                         
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                        
                       
                         ( 
                         
                           
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         [0000]    where, n thermal  is the thermal electron density in the conduction band; n pe  is the light excited electron density in the conduction band; h rod  is the height of the ZnO nanorod; R rod  is the radius of the ZnO nanorod; r b  is the Bohr radius of ZnO; and λ mfp  is the mean free path of electrons in ZnO. 
         [0023]      FIG. 2C  shows an example of a contour plot of electron density of a ZnO nanorod in a dark environment at 300K as a function of the radius and the height of the ZnO nanorod. When the radius or height is small enough, the dark electron density will arrive at a constant value, i.e., the thermal electron supersaturates. The curve  233  in FIG. 2D, as well as the dotted line  233  in  FIG. 2C , shows that the electron density changes with the height of ZnO nanorod with a fixed radius of 25 nm. It was found that when the height of the 25 nm ZnO nanorod decreases to 43 nm or below, the electron density becomes a nearly constant supersaturated value. The movement of electron transition between the valance and the conduction bands of ZnO is a dynamic equilibrium process. Under applied electrical potential, once electrons in the valance band are excited into the conduction band, they will move along the electrical field. Because the height of a short ZnO nanorod is small enough compared with the mean free path of electrons, the excited electrons do not have the opportunity to collide and fall into the valance band to recombine with holes. In other words, all electrons that can be excited have been excited into the conduction band and the valance band has no more electrons for further light excitation. Therefore, the illumination and dark conditions are almost the same in terms of electron density for short ZnO nanorods. 
         [0024]    The ratio of ZnO nanorod resistance under illumination and in a dark environment at 300K may be described as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       r 
                       light 
                     
                     
                       r 
                       dark 
                     
                   
                   = 
                   
                     
                       
                         
                           n 
                           thermal 
                         
                         + 
                         
                           n 
                           pe 
                         
                       
                       
                         
                           ( 
                           
                             1 
                             - 
                             
                               
                                 r 
                                 b 
                               
                               
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                                 rod 
                               
                             
                           
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                          
                         
                           ( 
                           
                             
                               n 
                               thermal 
                             
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                                 2 
                                  
                                 
                                   λ 
                                   mfp 
                                 
                                  
                                 
                                   n 
                                   pe 
                                 
                               
                               
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                                 rod 
                               
                             
                           
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                     . 
                   
                 
               
               
                 
                   EQN 
                   . 
                   
                       
                   
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         [0000]      FIG. 2E  illustrates a plot of the ratio for ZnO nanorods with a radius of 25 nm. From EQN. 2, the ZnO nanorod with a 25 nm radius can produce a stable and detectable resistance ratio when the length is longer than 500 nm. The photoelectric effect in ZnO nanorods will be undetectable when their physical sizes get too small. To overcome this limitation and significantly reduce the photoelectric pixel size, the height (or length) of the ZnO nanorods may be increased as compensation while keeping the radius small. 
         [0025]    A light spot position detector (LSPD) based upon the 3D PAD results will now be presented. The LSPD uses vertically aligned ZnO nanorods as 3D pixels, which results in both the ultra-high nanoscale resolution and outstanding photo detection performance. Referring to  FIG. 3A , shown is a schematic illustration of an example of the structure and operation of a sandwich-structured 3D PAD  112  ( FIG. 1 ) in the LSPD  300 . In the example of  FIG. 3A , one or more high density arrays of aligned nanorods  303  are perpendicularly grown between top and bottom electrodes,  306  and  309  respectively. For example, millions of vertically aligned nanorods (or nanowires)  303  can form nanoscale channels to confine the excited electrons within the volume of the corresponding nanorods  303 , thereby preventing lateral dispersion of the excited electrons. The top and bottom electrodes  306  and  309  can be parallel plates and/or conductive substrates. The top electrode  306  and/or the bottom electrode  309  allows at least a portion of a light spot  312  to pass through and illuminate a corresponding portion of the ZnO nanorods  303 . A plurality of resistance monitoring circuits  315  including a voltage supply  315   a  and a current monitoring device  315   b  are connected between the top electrode  306  and bottom electrode  309  at different locations around a sensing area of the LSPD  300  (e.g., connection points α and β). 
         [0026]    When the light spot  312  illuminates a portion of a sensing area of the LSPD  300 , the resistance seen by each independent resistance monitoring circuit  315  will be different due to the photoelectric effect of the ZnO nanorod(s)  303  under light excitation. By changing the location of the light spot  312 , the resistances of both resistance monitoring circuits  315  will change accordingly.  FIG. 3B  shows an example of an equivalent circuit for a resistance monitoring circuit  315  connected to the LSPD  300 . It is a serial and parallel hybrid resistance combination modeling the variable resistance of the ZnO nanorods  303  (R r ) and electrodes  306  and  309  (R f ). By measuring the current, and thus the resistance, with both resistance monitoring circuits  315 , the position of the light spot  312  can be determined. 
         [0027]      FIG. 3C  schematically illustrates how the position can be identified using the resistance monitoring circuits  315 .  FIG. 3C  includes a top view  321  of a LSPD  300  with a sensing area having a size of, e.g., 3×3 mm. The dashed lines are equal-current contour lines (about connection point α) seen by the resistance monitoring circuit  315  connected at α. As the position of a light spot  312  moves across the LSPD  300 , the resistance varies in a predictable way. For example, the resistance verses light spot distance curve  324  illustrates an example of the variation in resistance seen by the resistance monitoring circuit  315  as a 0.5 mm light spot  312  moves over a 2 mm distance on the top surface of the LSPD  300  from position  327   a  to  327   b . The other resistance monitoring circuit  315  connected at β sees similar equal-current contour lines about the connection point β. The intersecting point of two equal-current contour lines seen by the two resistance monitoring circuits  315  exactly locates the light spot position. 
         [0028]      FIG. 3D  illustrates contour surface plots of currents with a light spot  312  moving over the surface of a LSPD  300 , having a sensing area of 2×2 mm, with resistance measuring circuits  315  connected at α and β. The lower contour surface  340   a  corresponds to the current seen by the resistance monitoring circuit  315  connected at α and the upper contour surface  340   b  corresponds to the current seen by the resistance monitoring circuit  315  connected at β. The solid and dashed lines in the x-y plane of  FIG. 3D  are equal-current contour lines corresponding to the contour surfaces  340   a  and  340   b , respectively, and illustrate intersecting points of the equal-current contour lines. In this way, resistance measurements obtained by the resistance measuring circuits  315  can be used to determine the position of the light spot  312  in x and y coordinates. 
         [0029]    Multi-cycle tests were performed on a fabricated LSPD  300  with a sensing area size of 3×3 mm.  FIG. 4A  is a photograph of the fabricated LSPD  300  with connection points α and β indicated. The inset image  400  of  FIG. 4A  illustrates its transparent feature.  FIG. 4B  shows an SEM image of a cross-section of the LSPD  300 , which includes two parallel common electrodes and the enclosed perpendicularly aligned ZnO nanorod arrays. The ZnO nanorods have a spacing of about 200 nm. The multi-cycle tests were performed by repeatedly moving a small light spot  312  (about 0.5 mm in radius) on the 3×3 mm LSPD  300  with a step of about 200 nm and measuring the currents responding to the light spot position.  FIG. 4C  shows a graphical representation of the system used for testing the LSPD  300 . The system includes a light source  403  that projects a light spot  312  onto the LSPD  300 . The light source  403  may be repositioned in predefined increments. An analyzer  406  provides resistance measurement circuits  315  ( FIG. 3A ) to determine the resistances seen at the connection points α and β. A computing device  409 , such as a computer, may be used to convert the resistance measurements into the position of the light spot  312 .  FIG. 4C  graphically illustrates how a light spot  312  can be moved to, e.g., 9 points in a 3×3 matrix over the sensing area of the LSPD  300 . The gray scale of the measurement position  412  illustrate the relative difference in current (and thus resistance) seen by the resistance monitoring circuits  315  at the two connection points, α and β. During testing of the LSPD  300 , a 0.5 mm light spot  312  was moved at increments of about 200 nm to obtain the resistance measurements. 
         [0030]    Referring now to  FIG. 4D , as the light spot  312  is moved to different locations, the two current monitoring circuits  315  determine the currents at the two connection points. As shown in  FIG. 4D , the distance between adjacent measurement positions  412  is about 200 nm. The gray scale of the measurement positions  412  illustrates the proportional relationship between the magnitudes of the currents when light illuminates the corresponding positions  412 . By acquiring the currents through the two independent resistance monitoring circuits  315 , a current matrix can be obtained, which includes the current measurements corresponding to the different positions of the light spot  312 . The example of  FIG. 4D  illustrates a 3×3 current matrix where the current-time plots  415  and  418  show the measured current at α and β, respectively, for each corresponding measurement position  412 . When the currents are measured in increments of 200 nm, the location of the light spot  312  can be identified with a resolution of 200 nm. 
         [0031]    After the measurements have been completed, contour plots, lookup tables and/or mathematical relationships of the currents can be generated and used to determine the location of a light spot  312 . An example of contour surface plots generated from measured currents is shown in  FIG. 4D . In the illustrated example, the currents were measured with the light spot moving with a step size of 5 μm over a sensing area of 45×45 μm. The lower contour plot  421   a  represents the currents measured at connection point α and the upper contour plot  421   b  is the currents measured at connection point β. The solid and dashed lines in the x-y plane of  FIG. 3D  are equal-current contour lines corresponding to the contour surfaces  421   a  and  421   b , respectively, and illustrate intersecting points of the equal-current contour lines. Because of the incremental movement of the light spot  312 , the contour lines are not smooth. 
         [0032]    ZnO nanorod arrays can be synthesized using a low temperature hydrothermal method, which enables better morphology control and flexible polymer substrate compatibility. The electrodes can be fabricated from, e.g., gold thin film (or other appropriate transparent conductive material) using, e.g., deposition methods. In some implementations, the ZnO nanorod arrays may be grown on the bottom electrode using a bottom-up method and then covered with the top electrode. A top-down method (or a combination of bottom-up and top-down methods) may also be employed for etching semiconducting photosensitive thin film to form the nanorod arrays, which could be also served as the raw building materials for the 3D PAD  112  ( FIG. 1B ). The characteristics of the nanorod arrays and electrodes enable the 3D PAD  112  to possess flexibility, lower manufacturing cost, and tailoring convenience for a multitude of light spot detection applications. LSPDs using 3D PADs have low-cost, low-noise, and high-resolution characteristics. More importantly, such LSPDs can be mass produced for direct and wide applications due to an easy bottom-up production process. 
         [0033]    Referring to  FIG. 5A , shown are illustrations of examples of the structure and operation of a flexible and sandwich-structured LAPD  500  attached to an optical lens  503 . The light spot position through the lens  503  can be directly detected by the attached LSPD  500 , which includes two parallel electrode layers with high-density ZnO nanorods that are grown in-between the electrodes using a hydrothermal method. The ZnO nanorods are substantially perpendicular to the electrodes as shown in the SEM image  506 . The position at which the light spot  512  goes through the lens  503  can be determined from the current measured by two independent resistance monitoring circuits (not shown). The inset image  509  of  FIG. 5A  is an image of the as-fabricated LSPD  500  on a lens. The LSPD  500  was made of ZnO nanorod arrays grown over a large area (2 cm 2 ) on an arc-sharp Polydimethylsiloxane (PDMS). The LSPD  500  possesses high transmittance (as illustrated in  FIG. 5B ) and flexibility (as shown in  FIG. 5C ). 
         [0034]      FIG. 5D  illustrates contour surface plots of currents with a light spot  512  moving over the surface of the circular LSPD  500 . The contour plots  515   a  and  515   b  are of currents from two independent monitoring circuits connected at different points along the edge of the sensing area, where the light spot moved on a 1.5 cm×1.5 cm surface area with an incremental step of about 0.15 cm during calibration. The voltage applied to the LSPD  500  was 10V and the light spot  512  had a radius of about 0.5 mm. The lower contour surface  515   a  corresponds to the current seen by the resistance monitoring circuit connected at α of  FIG. 5B  and the upper contour surface  515   b  corresponds to the current seen by the resistance monitoring circuit connected at β of  FIG. 5B . The solid and dashed lines in the x-y plane of  FIG. 5D  are equal-current contour lines corresponding to the contour surfaces  515   a  and  515   b , respectively, and illustrate intersecting points of the equal-current contour lines. In this way, resistance measurements obtained by the resistance measuring circuits can be used to determine the position of the light spot  512  in x and y coordinates. 
         [0035]    In order to break the resolution limitation of prevailing 2D thin-film PSDs, a LSPD including a 3D PAD with 3D pixels has been developed using grown ZnO nanorod arrays. A fabricated LSPD has a measured resolution of 200 nm, which may be further reduced to resolutions of 10 nm or less. The 3D PAD of the LSPD includes arrays of, e.g., millions of nanorods, the size of which may be adjusted to provide improved resolutions. The volume of the photo sensitive material can be adjusted to avoid undetectable photoelectric characteristic due to thermal electron supersaturation. Furthermore, the 3D PAD made using a bottom-up method, which has significant advantages in fabrication, resolution, power consumption, etc. Furthermore, synthesizing ZnO nanorods on flexible substrates, enables the LSPD to be used in a wide variety of applications in flexible electronics. Direct application of the LSPD including a 3D PAD can have universal applications in industry, such as in precision machining and precise position control servo systems; in military applications such as, for example, target detecting and locating; and in daily life with, for instance, high dense media technology, etc. 
         [0036]    It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims. 
         [0037]    It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of “about 0.1% to about 5%” should be interpreted to include not only the explicitly recited concentration of about 0.1 wt % to about 5 wt %, but also include individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. The term “about” can include traditional rounding according to significant figures of numerical values. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ to about ‘y’”.