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The presence of a strong,changing,randomly-oriented,local electric field,which is induced by the photo-ionization that occurs universally in colloidal semiconductor quantum dots(QDs),makes it difficult to observe the quantumconfined Stark effect in ensemble of colloidal QDs.We propose a way to inhibit such a random electric field,and a clear quantum-confined Stark shift is observed directly in close-packed colloidal QDs.Besides the applications in optical switches and modulators,our experimental results indicate how the oscillator strengths of the optical transitions are changed under exteal electric fields.