Module -5 _ Unit-2 
                  Scanning Probe Microscopy (STM, AFM) 
                  Scanning probe microscopy was discovered in early 1980s, when it splendid the world by producing the 
                  first real-space images of surfaces with an unprecedented resolution of atomic-scale. Presently, SPM is 
                  one of the most employed technique in various disciplines which include fundamental surface science, 
                  routine  examinations  of  the  surface  roughness,  and  three-dimensional  imaging  (from  the  atoms  of 
                  silicon to microscale projections on the surface of a living cell). SPM belongs to the branch of microscopy 
                  which produce high definition images of a surface using a physical probes that scan the sample. This is 
                  done by keeping the probe stationary and moving the sample or vice versa. The collected data and its 
                  variation across the sample are employed to produce the sample’s image. 
                  The resolution of such an image depends on the sample, the control that one has on the movement on 
                  the tip/sample, and the inherent nature of the data. 
                  Surface of solids can be mapped showing structure of crystalline surfaces, distribution of point defects, 
                  structural defects like step, etc. 
                  The basic components of a SPM include probe and scanner, these are discussed in detail in the following 
                  sections. 
                  1.  Probe  
                  As the distance between two objects is decreased to very small values, a variety of interactions occur 
                  between them at atomic scale. These interactions form the basis of SPM. The probe is very sensitive 
                  towards these interactions and is configured to detect them. In SPM, this probe is brought very close to 
                  the sample being imaged. The magnitude of the interaction between probe and sample depends on the 
                  distance between them. This gives an SPM the ability to precisely map the surface topography of a 
                  specimen. 
                      Probe Tips of SPM 
                  The type of probe required depends upon the type of SPM being used. The probe of SPM is having a 
                  very sharp apex. The resolution of SPM is determined by the apex of the probe, such that the sharper 
                  the probe is, better is the resolution. For many cantilevers based SPMs (e.g. AFM) the entire cantilever 
                  as well as the probe are produced via etching, most commonly from silicon nitride (SiN). The conductive 
                  probes for STM are usually constructed from platinum/iridium wire. 
                  2.  Scanner 
                  A piezoelectric  ceramic  is  used  to  provide  the  precise  positioning  control  necessary  to  all  the  SPM 
                  scanners. The application of voltage causes a corresponding change in the geometry of the piezoelectric 
                  ceramic.  This  voltage,  in  turn,  is  proportional  to  the  resulting  mechanical  movement.  In  SPM,  this 
                  piezoelectric scanner undergoes controlled and predictable bending, expansion as well as contractions. 
                  Thus, the probe-sample gap and the position of the probe, both can be controlled via this scanner. SPM 
                  scans the sample surface in a raster fashion. To create an image, specific values (depending upon the type 
                  and mode of SPM) are recorded while the probe is rastered over the sample surface. A map is created 
                  from these values which produces the final image of the sample surface. 
                   
            
           Let us assume ‘P’ represents the interactions between the tip and the sample surface. P can be used in the 
           feedback system (FS) only if there exists a sharp and unique dependence of P on the gap between the tip 
           and sample, i.e., P = P(z). The feedback system controls the distance between the tip and the sample 
           surface. The feedback system in a SPM is schematically shown in Figure 1. 
                                                                              
                    Figure 1 Schematic representation of the feedback system as used in a SPM. 
            
           A default value is set for the parameter P, such that P = P . The feedback system then maintains this 
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           value of P. As the gap between the tip and sample varies, the value of P changes from its equilibrium 
           value P . The change in P (or ∆P) is fed to the feedback system, which then amplifies it and feeds a piezo 
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           transducer (PT). PT, in turn, controls the gap between tip and sample. PT uses ∆P to alter the gap 
           between tip and sample, and brings this gap to its initial value, making the differential signal (∆P) close 
           to zero. As the tip is moved across the surface of the sample, its topography alters the interaction 
           parameter P. Feedback system responds by restoring the default value of the tip-sample separation (i.e., 
           P = P ) in realtime. Thus when tip reaches a point x,y at the sample surface, the signal V(x,y) fed to the 
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           transducer is proportional to the departure of the sample surface from the ideal plane X,Y(z = 0). As a 
           result, V(x,y) can be used to map the sample surface, thereby obtaining the SPM image. Apart from 
           visualising the surface of the sample, SPM can also be used for determining various properties of the 
           sample, these include, mechanical, electrical, magnetic, optical, etc. 
           Types of SPM- 
           There are two forms of SPM: 
                1.  Scanning Tunneling Microscope (STM) 
                2.  Atomic Force Microscope (AFM) 
                    
            
        
       1.  Scanning Tunneling Microscopes (STMs) 
       STM was devised by Gerd Binning and Heinrich Rohrer in 1981, while working at IBM. They prepared the 
       first instrument to produce atomic-scale real-space images of surfaces. They were awarded the Physics 
       Nobel Prize for this work.The phenomenon of electron tunneling is exploited to acquire an image of the 
       sample surface. This utilizes the principle of vacuum tunneling. Here, the tunneling probe and a surface 
       are brought near contact, at a small bias voltages.  
       If two conductors are held close together, their wavefunctions can overlap. The electron wavefunctions 
       at the Fermi level have a characteristic exponential inverse decay length, K expressed as: K= √(8mφ)/h, 
       here, m is the mass of an electron, and φ is the height of local tunneling barrier, also termed as the 
       average work function of the tip and the sample. 
       Tunnelling  current  varies  exponentially  with  the  distance  between  tip  and  sample,  such  that  for  a 
       change of an angstrom in the tip-sample gap, changes the tunnelling current by an order of magnitude. 
       This makes STM extremely sensitive towards any changes in tip-sample gap. The sample surface can be 
       imaged both vertically and horizontally with atomic scale resolution. 
        
                                                 
            Figure 2 Schematics of electrons tunneling through a potential barrier in STM. 
                              
       The tunneling current, I decays exponentially with the gap as IαVe^-√ (8mφ)/h2d, where d is the tip-
       sample gap and φ is the work function of tip. 
       The tunneling current is a result of the overlap of electronic wavefunction of the tip and the sample. In 
       this device, a tip is brought closer to the specimen so that the electrons can tunnel across the vacuum 
       barrier. The position of the tip is adjusted by two piezoelectric scanners, with x and y control. The z axis 
       position  is  continuously  adjusted,  taking  feedback  from  the  tunneling  current  so  that  a  constant 
       tunneling current is maintained. The position of the z- axis piezo therefore reproduces the surface of the 
       material. The other mode of imaging is by modulating the tip at some frequency and measuring the 
       resulting current modulation. STM allows the investigation of local structures. 
       STM provides information on the local density of states (DoS). The DoS is the quantity of electrons 
       existing at particular entry levels in a material. Keeping the tip-sample separation constant, a measure of 
       change in current due to bias voltage can probe the local density of states of the specimen. 
        
                     
                    Electrons can tunnel to states which are present in the sample or tip. When the tip is negatively biased, 
                    electrons from the tip tunnel from the occupied states of tip to the unoccupied states of specimen. 
                    Thus, the spatial resolution achieved by using these tips is in atomic scale. 
                                                                                                                                               
                                             Figure 3 Atomic resolution in a scanning tunnelling microscope. 
                     
                     
                    Advantages of the STM: 
                        STMs  provides  a  three-dimensional  profile  of  the  surface,  which  allows  examining  multiples 
                         characteristics of the sample, such as roughness, surface defects, etc. 
                        It  also  allows  measurement  of  various  properties  of  the  samples,  such  as  electrical,  mechanical, 
                         optical, etc. 
                        STMs are versatile and are operable under ultra-high vacuum or ambient conditions. The samples 
                         investigated can be solid, liquid and gas. 
                        STM can be operated in a wide temperature range: form 0K to few hundred Celsius. 
                        Lateral resolutions of the order of 0.1 nm, and depth resolution of 0.01 nm can be obtained. 
                        The atomic arrangement on the surface can be obtained by measuring the DoS of the surface atoms. 
                        
                    Disadvantages of STM: 
                        STMs are complex to operate effectively. It is highly specific technique, requiring great skill and 
                         precision. 
                        The surface must be very stable and clean.