A shift in the spectra means some physical properties of your sample have changed. Depending on which spectroscopy type you use, you may probe different properties. Therefor it is first important to know, to which property your signal may relate to. This is usually the most difficult part and depends a lot on your material system and the spectroscopy method you use. Therefor, you always have to be careful, as a lot of effects can cause shifts or even the appearance of new bands. So it is very difficult, too find a general answer. Just take these examples into account:

UV-Vis spectrosocpy:

Here, you usually see the signatures of electronic transitions (band-band transitions or HOMO-LUMO transitions for molecules). If those show shifts, your electronic structure have been modified by your doping material. Also selection rules may be relaxed by doping due too lowered symmetry. Dopant atoms can now act as activation centers for scattering. If your spectral signatures are related to plasmonic resonances, a shift means your dielectric functions has been modified by doping.

Raman spectroscopy:

Raman spectroscopy is usually sensitive to vibrational states. Here any shift means the eigenfrequencies of the system changes. This can relate to larger masses of atoms, different force constants and different bond length. Shifts of Raman bands can also be related to size of nanoparticles, which usually causes a red-shift (see Phonon confinement model). Free charge carriers can also couple to longitudinal optical modes, an effect known as longitudinal optical phonon-plasmon mode (LOPC). Here, free charge carries can cause of blue-shift of LO modes.

Dear Raj Kumar,

      The Uv-Vis spectrum of the plasmonic nanostructures can indicate the resonant properties of the nanostructures. This maybe can be explained with the help of some concepts in the field of plasmonics. Surface plasmon (SP) is collective charge oscillation that occurs at the interface between metallic conductors and dielectrics. As you know, free elcetrons of metallic nanoparticle (MNP) exsis in the surface of MNP. When incident light interact wiht this NP, the free electrons in its surface will oscillate. In fact, the optical properties of metallic nanostructures depend on their shapes, materials, sizes and the surroundig envirenment.

       During adsorption or doping progress, the plasmon coupling between different plasmonic nanostructures or surrounding envirenment changements induced by the doped nanoparticles all can have a distinct effect on the surface plasmon resonance properties of the system, leading to the changes in energy levels and resonant wavelengths. Therefore, the corresponding blue shift or red shift appear.

      Although this answer is not very detailed, I hope it will be helpful, if so, it will be a pleasant thing.

Tingzhuo

A shift in the spectra means some physical properties of your sample have changed. Depending on which spectroscopy type you use, you may probe different properties. Therefor it is first important to know, to which property your signal may relate to. This is usually the most difficult part and depends a lot on your material system and the spectroscopy method you use. Therefor, you always have to be careful, as a lot of effects can cause shifts or even the appearance of new bands. So it is very difficult, too find a general answer. Just take these examples into account:

UV-Vis spectrosocpy:

Here, you usually see the signatures of electronic transitions (band-band transitions or HOMO-LUMO transitions for molecules). If those show shifts, your electronic structure have been modified by your doping material. Also selection rules may be relaxed by doping due too lowered symmetry. Dopant atoms can now act as activation centers for scattering. If your spectral signatures are related to plasmonic resonances, a shift means your dielectric functions has been modified by doping.

Raman spectroscopy:

Raman spectroscopy is usually sensitive to vibrational states. Here any shift means the eigenfrequencies of the system changes. This can relate to larger masses of atoms, different force constants and different bond length. Shifts of Raman bands can also be related to size of nanoparticles, which usually causes a red-shift (see Phonon confinement model). Free charge carriers can also couple to longitudinal optical modes, an effect known as longitudinal optical phonon-plasmon mode (LOPC). Here, free charge carries can cause of blue-shift of LO modes.

simple that we have studied in our pg time if your moitey contain chromophore you will get red shift. 

Along with the above answers,ist keep in mind that whenever higher energy absorption takes place due to exciton/plasmon/polaron/.....,it leads to a shift towards smaller wavelength i.e blue shift and viceversa

Research gate experts give their own experience view for questions thats more inspirable than our learned education.

Thanks for your nice clarification Professor. Tingzhuo Liu, Michael Rüsing, Kalagi

Mr. Kumar,

The most importan aspects are that evaluating even the fluctuations of the spectroscopic bands, including bathochromic, hypsochromic, hyperchromic and more effects or resp. high- or low-frequency shifting, you can achieve information about the intermolecular structural changes and/or perturbation of theelectronic states due to the environmental factors [effect of the continuum]. Here have been added also effects of the T, and P.

Please find as attachment few examples about the intramolecular factors causing for the structural changes, which have significantly effected the IR-, Raman, and UV-profile of the compounds studied with the corresponding refs as well as an example on the continuum effect on the UV-spectra. 

The problem of blue and red shifts in vibrational spectroscopy is described in detail in my review attached.

@Michael Rüsing.

Dear madam will  you please  avail me the reported result of your comment  : Free charge carriers can also couple to longitudinal optical modes, an effect known as longitudinal optical phonon-plasmon mode (LOPC). Here, free charge carries can cause of blue-shift of LO modes.

The explanations here were very handy for me. Thank you.

how about the shift in PL with the addition of dopant? it also gets broadened

what exactly is phonon confinement? Please explain

for more information, you can refer this paper

Article Effect of phonon confinement on photoluminescence from collo...

very useful information